What do these symbols & annotations in medieval and related texts mean?

Most of the symbols you will encounter in that way are based on editing conventions more or less agreed upon through tradition in usage. Most of these will either follow the Leiden convention for symbols or have to be explained somewhere in the footnotes or glossaries of the editions you'll read. They are used almost universally for the transcription, description, critical editions or quotations of ancient, medieval or even modern texts, inscriptions and so on.

As @sempaiscuba covers the examples, here are some of the most important sigla for this system:

[ ]     Square brackets indicate that the bracketed section on the original inscription is damaged and no longer legible, or at least very difficult to read, and has been supplemented by the editor for the printed publication. The restored characters are reproductions of the unreadable original that have been classified as probable to a degree of probability bordering on certainty.
[...]   Dots on the line indicate the determinable number of non-reconstructible letters (in this case three).
[- -]   horizontal lines indicate an indeterminable number of non-reconstructible letters.
( )     Round brackets indicate that the bracketed part of a word was omitted in the original, i.e. that the term was abbreviated. The content enclosed in such brackets completes the abbreviation used.

        Example: P(ontifex) M(aximus) means that instead of the written Pontifex Maximus only PM can be found in the original inscription.
        Example for the use in a translation: Smikylion (son) of Eucalides. (In ancient inscriptions, the name of a person is often followed by the name of his father in the genitive and without a more precise explanation of the relationship, so this must be supplemented in the translation.
< >     angled brackets indicate that the editor has corrected an error in the original inscription (for example, inadvertently omitted letters, spelling mistakes, or an erroneous number).
        Sometimes the erroneous part of the text is simply replaced by the correction (i.e. "C<ae>sar", although the original erroneously says "Ceasar") - then a reference to the original spelling must be made in the commentary on the edition.
        Sometimes, however, both the wrong and the corrected spelling are indicated within the angle brackets, e.g. according to the format "C<ae=EA>sar".[12]
{ }     Curly brackets surround text that the editor deletes as superfluous (for example, words or parts of words that are accidentally written twice).
ạḅc̣     A dot below the letter indicates that the original letter is only partially preserved and that it is not clear from the still visible lines (even if it can be reconstructed with great certainty due to the preceding and/or following letters).
...     dots on the line indicate the number of assumed non-reconstructible letters (Greek and papyrological).
+++     plus signs on the line indicate the number of assumed non-reconstructible letters (Roman)
[[abc]] In the scientific nomenclature, the double clasping of a text section is referred to as shaving, which means that the clasped section was intentionally removed from an inscription in ancient times. The reasons for this are mostly politically motivated: for example, the Roman emperor Caracalla had the name of his brother and co-regent Geta erased from inscriptions after he had murdered him, which both named as equal rulers. This measure is called Damnatio memoriae.
        If, despite shaving, parts of a letter are still recognizable, a dot is placed under them: [[ạḅc̣]]
v
vv
vacat   for "empty" indicates an unmarked position in the text witness. The size of the blank field can be indicated by the number of letters that could have been left there according to the font size.
        If the editor suspects an unmarked place, which however cannot be proved or cannot be proved with certainty due to the state of preservation of the original, this can be indicated by [vacat] or ṿ (i.e. by square brackets or a dot below the letter).
|       Vertical lines mark the beginning of a line if the text is not printed with the original line breaks.
||      Vertical double strokes mark the beginning of every fifth line for the sake of clarity.


...             Illegible letters, not restored by the editor (extent known or approximately known, one dot per letter). Above example: three illegible letters. Specific numbers between two dashes used for extended areas, instead of individual dots per letter. In the latter case, approximation, if any, may be expressed with plus-minus sign replacing the first dash.
[...]           Letters missing, not restored by the editor (extent known or approximately known, one dot per letter). Above example: three letters missing. Specific numbers between two dashes used for extended areas, instead of individual dots per letter. In the latter case, approximation, if any, may be expressed with plus-minus sign replacing the first dash.
[ or [ ] or ]   Letters missing, not restored by the editor, extent unknown.
[abc]           Letters missing, restored by the editor.
⟨ ⟩ or ***      Letters erroneously omitted by the text, not restored by the editor.
⟨abc⟩           Letters erroneously omitted by the text, restored by the editor.
a(bc)           Abbreviation in the text, expanded by the editor. Doubtful expansion should be expressed with a question mark before the closing parenthesis: a(bc?).
{abc}           Letters considered erroneous and superfluous by the editor. If illegible, an individual letter is expressed by a single dot each. Doubtful letters are marked by a subscript dot.
⟦abc⟧           Rasura: a deletion which can be restored. In this example, the letters abc were deleted, but are still legible or can be restored from context. Deletions may be specified in the apparatus as well.
\abc/           Interlinear addition of letters in the text itself. In this example, the letters abc were added between lines. These sigla are used when interlinear text is otherwise difficult to represent as such typographically.