The term out of position refers to a player who is acting before their opponent. This means that they can’t see how their opponent will act and can’t adjust their strategy based on that, which puts them in a disadvantageous position.
For example, players in the small blind seat are always out of position. They have to act first on every street after the flop, giving their opponents information they can use against them. In contrast, the button is always in position on all consecutive streets after the flop.