What does merit (puṇya/puñña) actually mean?

Pleasantness is a synonym for good/merit (puṇya/puñña).
Mettasutta

“Mā, bhikkhave, puññānaṃ bhāyittha. Sukhassetaṃ, bhikkhave, adhivacanaṃ yadidaṃ puññāni.."

Bhikkhus, do not fear to do good. Pleasantness is a synonym for good. Bhikkhus, I know of enjoying the results of pleasing and agreeable good, done long ago...

Merit is any thought, deed or word based on generosity, compassion and / or wisdom. When we are born we bring our store of Karma from your past lives. During this life we consume the results of this store as well as accumulate new Karma.

You know you are accumulating Karma when you delight on experiences that you perceive and favorable, averse to experiences that you perceive as unfavorable and not aware of the arising and passing nature of experiences that are neutral. In other words you are clinging or craving to sensual input and clinging to conceptualisations that follow.

Mr. Concept,

• Is merit something that follows my thoughts and actions as a consequence?

Yes, like demerit is: Dhp 1 & 2 when you think and do.

• Or is merit more like an acquired disposition to act and think in certain ways?

Yes, like demerit is: Dhp 1 & 2 when you ponder about thinking or doing

• Or perhaps it's both an acquired disposition and a consequence?

Yes, like demerits is: Dhp 1 & 2 and you meet even the fine nuance that makes the Buddhas teaching different for other kamma ideas. That it is the past intentions and the present intention in the moment, which forms you future experience future.

• Are we born with a certain "amount" of merit due to karma?

You are born because of kamma, yes.

• How does a person know that s/he is accumulating merit?

Only when a person is certain trained, he can reflect it such was at least meritorious. Puñña means always giving, letting go and leads always to release. So when ever there comes up a certain release and relaxation, then there was merit as its cause. Its like having done work. If you have done it correct and right, you lay down relaxed. Of course you can lay down even if not done or not correct, but that possibility is just a making use of previous fruits, credit(s). See as explaining the Lonaphala Sutta: The Salt Crystal

A nice overview is "Ten Ways of Making Merits" and if you like to understand it better, Atma suggest you to get into a little "Abhidhamma in Daily Life", by Ashin Janakanbhivamsa, which is also good to read and with samples.

What you, how ever should not do, is to try to trace kamma up and down, such, as the Buddha told, will make you crazy. On the matter how kamma actually works and is used to escape, a read of Kamma & the Ending of Kamma and Kamma, A Study Guide by Ven. Thanissaro Bhikkhu, might be useful.

And since you are doing here merits "listening to the Dhamma", as for every merits, that you are sure to make not just an exchange or keep anything, don't forget to share your merits, dedicate them to ever living being, which is able to rejoice with your merits you have done. Dedication of Merit

(Note: this answer has not been given with the agreement to be means of trade or the purpose of/for trade and/or keep people trapped and bound. How you handle it lies in your sphere, but does not excuse the deed here either.)

Merit is the predisposition to positive experiences due to previous thoughts, words, and acts, driven by intention. It conditions future experiences and can be created, accumulated, expended, lost, and transferred to others, just like money (the analogy is not accidental, and has been associated by others than myself with the fact that mercantilism was taking hold during the Buddha's time and that the many of the Buddha's followers came from the new merchant classes). The word simply means virtue, righteousness, sanctity, etc. So long as one has unfruited karma, one has merit and demerit and this conditions rebirth, especially human rebirth, which is itself regarded as the result of merit. Therefore, the first task of a human being is to ensure that one accumulates a store of merit in order to be reborn as a human being at least. One must ask, why should one care since one will (most likely) not remember one's previous lives if one is reborn as a human being (a trait we share with animals). However, hungry ghosts and hell beings do appear to remember their previous lives, based on the suttas. You know that you are acquiring merit by always acting with righteous intention, esp. (but not only) by following the Five Precepts (pansil).

There is here a 119 pages transcript of a teaching given by Khenrinpoche Geshe Chonyi, in which he answers that exact same question. Search for 'what is merit' and you will find the answer, page 26. Although Khenrinpoche's answer is not extremely precise (it that it might not stand debate) there are greatly illuminating points:

• In one way, merit and virtue are the same. In general, virtue is 'virtuous karma', for it is a cause of the fruitional effect of happiness [in the aspect of pleasant feelings, and so forth].

Additionally, a collection of merits is:

1. The virtuous imprint that is left in the mind
2. A non-associated composition factor (like karmic imprints, so it stands to say they are the same in general)
3. A cause of the Form Body [whereas the collection of wisdom results in the Truth Body]

He also adds:

I would think that, the meaning of the word accumulate in the phrase accumulate the collection of merit would mean that, doing something repeatedly such that, a real impression, mark, imprint is left on the mind . This is the meaning of to accumulate .

Being virtuous karmic imprints, it is produced by every virtuous intention (like for instance the virtuous intention to give, which is generosity) as well as any virtuous bodily or verbal action. Being virtuous karmic imprints, it must also have the same 4 types of result, one of them being the resultant tendency concordant to the cause. It means you will have a seemingly natural tendency, and liking for, say, practicing generosity and so forth.