Definition
Position in poker refers to where a player sits relative to the dealer button, which determines the order of action in each betting round. Acting last (being "in position" or IP) is a significant strategic advantage: you see what every other player does before you must act, giving you more information to make optimal decisions.
The positions in a standard game, from earliest to latest to act post-flop: Small Blind, Big Blind, UTG (Under the Gun), UTG+1, Middle Position, Lojack, Hijack, Cutoff, and Button (dealer). The Button acts last post-flop in every round and is therefore the most profitable position at the table.
Being "out of position" (OOP) — forced to act before an opponent — is a significant disadvantage that requires compensation through tighter ranges and more cautious play. The value of position is reflected in statistics: players consistently win more from the Button than from any other position.
Example
On the Button with T♠9♠ in a 3-bet pot, you have significant positional advantage. You can call the continuation bet on a Q♣6♦2♥ flop and reassess on the turn, gathering information about your opponent's hand strength before committing chips.