To suck out on someone at a poker table means to beat them in a hand despite them being a heavy favorite at one point. A suck-out has a similar meaning to a bad beat, as both expressions describe a situation where a hand that was very unlikely to win ends up a winner.
A poker suck-out usually happens in an all-in situation, where both players have committed their stacks, and there is no further betting action.
However, you can sometimes hear players and commentators talk about someone sucking out in an active hand, starting with a very weak holding and backing their way into a monster like a straight, flush, or even a full house to beat a hand like a flopped set.
An example of a really bad suck-out would be two players ending up all in on the board of A 9 T. One player has T9 for two pairs, and the other has 89. The player with 89 is a big underdog, as there is no single card that can give them a winner. However, the turn comes a 6, and the river comes a 7, giving them an unlikely straight.