When you act later, you get more information from your opponents’ actions. There is also relative position, where you are IP over one player, but OOP against another. Otherwise, you are out of position (OOP).
If you are the last player to act post-flop, then you are in position (IP).
After the flop is dealt, the action starts from the small blind position. In standard games, there is only one small and one big blind, and first person to act before the flop is the early position player. Some casinos and online poker sites allow players to put additional actions into the game before the cards are dealt, like adding single or multiple additional blinds (at least twice the big blind amount) called straddles and table-wide bets called antes. The big blind is always the minimum betting amount, and the small blind is half that same amount. The action always moves clockwise, in betting rounds or when moving the blinds on position between each hand.
