Penchan is an edge wait made of two sequential tiles at the boundary of a suit: 1-2 waiting on the 3, or 8-9 waiting on the 7. Unlike a ryanmen wait, one side is blocked by the edge (there is no 0 and no 10), so only a single value completes the group.
In tenpai, a penchan wait accepts just one rank, at most four tiles of ukeire, and often fewer when copies already sit in the discards or in your own hand. Like kanchan and tanki, penchan grants 2 fu when it finishes the hand, a small compensation for how narrow it is.
A penchan has narrower maximum ukeire than a ryanmen. Improving it can speed up the hand, but the right choice also depends on shanten, yaku, dora, visible copies and discard danger. No universal rule says to keep it or break it.