Normally, a character can move through a space occupied by an enemy, but the enemy must be either helpless or at least two size categories smaller or larger than the character. The character provokes an opportunity attack from the enemy if the enemy isn't helpless, because the character leaves a threatened square to enter the enemy's space.
A character with the Underfoot feat can move through spaces occupied by an enemy without provoking an opportunity attack from that enemy, if the enemy is at least two size categories larger than the character. Specifically, leaving a threatened square to enter the enemy's space, moving between spaces occupied by the enemy, and leaving the enemy's space into an adjacent space, no longer provoke opportunity attacks from that enemy.
Characters with the Underfoot feat can still provoke opportunity attacks from enemies other than the one whose space they are moving through. They still can't end moves in another creature's space.[PH:206]