Everything the hexagram argues rests on this one stroke. Legge's translation gives a superior man of acknowledged merit who maintains his success to the end, with good fortune, and the two halves of that sentence are doing different work. Merit is acknowledged, so the achievement is public and not in question. What remains open is whether it will be carried to completion by someone who stays modest while carrying it, and the line answers that it will.
This is the only strong line in Modesty and the ruling line of the hexagram, standing at the top of the lower trigram Gen, correctly placed in an odd position. Its singularity is the point. Modesty in a hexagram of six yielding lines would be an easy virtue and would prove nothing; here there is exactly one position with real substance to be modest about, and the whole reading of Modesty depends on what that position does with it. Every other line in the hexagram is read in relation to this one.
Toil without claiming credit is the classical gloss on the phrase, and it explains why the good fortune is attached to the end rather than to the merit. Work that is acknowledged partway through invites its holder to stop and collect on it. The line describes someone who does not, and its promise is simply that the task reaches completion, which is the reward a person of this character would have wanted in the first place.