What does the first para of Lonaphala Sutta mean?

While I am not fluent in the exact translation, the translation offered is as follows:

Monks, for anyone who says, 'In whatever way a person makes kamma, that is how it is experienced,' there is no living of the holy life, there is no opportunity for the right ending of stress. But for anyone who says, 'When a person makes kamma to be felt in such & such a way, that is how its result is experienced,' there is the living of the holy life, there is the opportunity for the right ending of stress.

There is the case where a trifling evil deed done by a certain individual takes him to hell. There is the case where the very same sort of trifling deed done by another individual is experienced in the here & now, and for the most part barely appears for a moment.

Based on reading the above two paragraphs, it appears the meaning of the sutta is:

1. When the Holy Life has been lived, an act of evil kamma, performed due a lapse of mindfulness, does not necessarily bring an evil result, i.e. a result of suffering (which can only occur due to egoistic attachment or self-view).

2. When the Holy Life has been lived, where there is skill in mindfulness & wisdom, an act of evil kamma, performed due a lapse of mindfulness, only brings the result of vedana (feeling). Where a certain individual is developed in [contemplating] the body, developed in virtue, developed in mind, developed in discernment: unrestricted, large-hearted, dwelling with the immeasurable, the following occurs in relation to an act of evil kamma, in the here-&-now:

On cognizing a mind-object with the mind, he does not lust after it if it is pleasing; he does not dislike it if it is unpleasing. He abides with mindfulness of the body established, with an immeasurable mind, and he understands as it actually is the deliverance of mind and deliverance by wisdom wherein those evil unwholesome states cease without remainder. Having thus abandoned favouring and opposing, whatever feeling he feels, whether pleasant or painful or neither-painful-nor-pleasant, he does not delight in that feeling, welcome it, or remain holding to it. As he does not do so, delight in feelings ceases in him. With the cessation of his delight comes cessation of clinging; with the cessation of clinging, cessation of being; with the cessation of being, cessation of birth; with the cessation of birth, ageing and death, sorrow, lamentation, pain, grief, and despair cease. Such is the cessation of this whole mass of suffering.

MN 38

As was taught in AN 3.61:

Now it is for one who feels that I proclaim: ‘This is suffering,’ and ‘This is the origin of suffering,’ and ‘This is the cessation of suffering,’ and ‘This is the way leading to the cessation of suffering.’

Translation:

Bhikkhus, if someone says: "how many actions a person done are how many results a person get back". Bhikkhus, if that be so, there is no meditation for enlightenment and there is no apparent path to end suffering rightfully.

Bhikkhus, if a person says: "how many actions a person done, only results which possible to get back, are how many results a person get back". Bhikkhus, if that be so, there is no meditation for enlightenment and there is no apparent path to end suffering rightfully.

http://www.84000.org/tipitaka/read/v.php?B=20&A=6567&Z=6647&eng=metta_e

(I modified from that link.)

Why I translate like that?

I combine above titles with their explanation below. Because in ancient theravāda study system culture, sutta's context must relate each others. This is called tanti-bhāsā.

Here bhikkhus, a certain person doing a trifling evil action would pull him to hell. Here bhikkhus, a certain person doing a trifling evil action of the same nature would feel it here and now and mostly nothing would be apparent.

Bhikkhus, what kind of person doing a trifling evil action, on account of it, be pulled to hell?.

Here bhikkhus, a certain person is undeveloped, in body,2 virtues,2 in mind,[3] in wisdom,[4] is insignificant with a small self[5] living in unpleasantness. Bhikkhus, such a person doing a trifling evil action would pull him to hell (uppapajja-vedanīya; this life's and hell in next life's results of puthujana are possible to get back).

Bhikkhus, what kind of person doing a trifling evil action of the same nature would feel it here and now and mostly nothing would be apparent?

Here bhikkhus, a certain person (ariya) is developed, in body, virtues, in mind, in wisdom, is with a significant self immeasureable and living in pleasantness. Bhikkhus, such a person doing a trifling evil action of the same nature would feel it here and now (diṭṭhadhamma-vedanīya; this life's results of ariya are possible, but hell in next life is impossible) and mostly nothing would be apparent.

So, "how many actions, ditthadhamma-vedanīya and hell in upapajja-vedanīya, a person done, only results which possible to get back, are how many results a person get back". (puthujana can get both diṭṭhadhamma-vedanīya and hell in upapajja-vedanīya back, but ariya can get just diṭṭhadhamma-vedanīya back.)

Therefore buddha taught in Cakkhu Sutta:

4. “Monks, if anyone takes faith and is settled in these things (be sotāpanna-ariya), it is said he lives in conformity with faith, has entered the righteous way, the way of Great Beings, has gone beyond the field of ordinary man, and it is not possible that he is doing actions that lead to hell, the animal womb, or the sphere of ghosts be born there. It is not possible that he should die without realizing the fruits of entry in the stream of the Teaching.