It generally indicates that the cigar has aged well and will likely be even tastier than it was before. It doesn't seem to happen to dog rockets as far as I've seen.
If you talk to BeatDragon about this, he'll probably disagree. He claims that most cigars don't benefit from aging at all. Only cigars shipped young by corner-cutting manufacturers do.
I disagree. For example, my Oliva "O" Maddies with 5-yr, pre-roll, aged tobacco have benefited (mellowed) tremendously from humi time. Just ask Limoric about those.