Where you sit relative to the players still in the hand, and specifically whether you act after them. Acting last is the most durable structural advantage in the game, and it does not depend on cards, reads, or skill gaps.
The reason is information. In position you see what everyone else does before you commit anything. You can call a bet cheaply and take a free card when you miss, or raise when their sizing tells you they are weak. Out of position you make every decision blind and then find out what it cost. The same hand is straightforwardly more profitable when acted last, which is why seat-by-seat results in any large database form a staircase: the button prints money, the seats before it earn less and less, and the blinds — forced to put money in and then act first for the rest of the hand — lose.
The practical consequences are direct. Play more hands in late position and fewer in early position, and treat marginal holdings as position-dependent rather than as good or bad in the abstract: a suited connector that is a comfortable open on the button is a routine fold under the gun. When you are out of position, prefer lines that reduce the number of decisions you have to make first — betting to take the initiative, or checking with a plan for each of the sizes you might face, rather than drifting street to street and guessing.
Position is also why hand strength alone is a poor guide to whether to enter a pot.