The poker term “in position” refers to the player who is the last to act in the round. When a player is in position, they have an advantage over their opponents, as they’ve seen what every other player decided before them.
The player in the button seat is guaranteed to be in position in all betting streets unless they fold. Oppositely, the player in the small blind position, located to the button’s left, is always out of position, as they’re acting first in every round after the flop.
For example, if someone opens from the CO and you decide to 3-bet from the BTN, you are in a position against that player. If blinds fold and CO calls, he will be forced to act first after the flop, putting you in an advantageous position.