Punishment is hitting an opponent while they are recovering from using a move, and cannot block. This can be because the move missed due to spacing, duck or sidestep. But it can also be because the move recovers extremely slowly when blocked; these are known as "unsafe moves". Typically, a move that's -10 frames or worse on block is considered unsafe.
A move that's -15 or worse on block is extremely unsafe, because most characters can get a launch for blocking one.