In games where the relative position of a player to the dealer determines the order of play, the early positions are those who must act first. At a full ring table in Texas Hold'em, for instance, the first three positions after the blinds are the early positions, also called UTG (under the gun), UTG+1, and UTG+2.
The disadvantage of players in early position is that they must act early in the round and so they don't know what the other players will do. The more players that will act after a certain player, the heavier the disadvantage of information weighs upon him, and the more probable it is that a player behind him has a strong hand. It is more difficult to craft a game against opponents acting later than against those who have already acted.