Position
Where a player sits relative to the dealer button, and so how much information they have before acting: under the gun (first to act), middle position, the cutoff, and the button (last to act) are the four common reference points.
Acting later means acting with more information: every opponent who has already acted has revealed something about their hand before the player in position has to commit anything. That is why opening ranges widen steadily from under the gun through to the button, and why the same two cards can be a clear fold from one seat and a routine raise from another. Position does not change the cards; it changes how much of their value can actually be extracted.