Out of position (OOP)
Also called OOP
Out of position (OOP) means you have to act before one or more opponents on the flop, turn, and river, so you act first and decide without seeing what they do.
In Texas Hold'em, the order you act in is fixed by where you sit relative to the dealer button, and it stays the same on the flop, turn, and river. When you're forced to act before an opponent on those streets, you're out of position against them. The opposite is being in position, where you get to act last and see what everyone else does first.
This comes up on every hand you play, because someone is always first to act. The blinds are out of position against almost the whole table, and any seat acts before the players to its left. Being OOP is a disadvantage: you have to decide whether to check, bet, or fold without yet knowing what your opponent will do, while they get to react to you with full information.
For beginners, the simplest takeaway is that hands play tougher out of position, so it pays to be a little more selective about which ones you get involved with from early seats and the blinds. When you are OOP, leaning on a clear plan helps: you might check to your opponent and decide based on their action, or fire a continuation bet to take the lead. Tools like the check-raise exist partly to fight back from out of position.
Example
You're in the big blind with K-Q and call a raise from the button; on a flop of K-7-2 you must act first every street while the button always acts after you, so you're out of position the whole hand.