Complex sentence: Independence relations are modelled in the game tree as indistinguishability relations between game states with respect to a certain player: in other words, the player is not certain what previous moves (this can simultaneous).
Split into simpler sentences: Independence relations are modelled in the game tree as indistinguishability relations between game states with respect to a certain player. In other words, the player is not certain what previous moves (this can be simultaneous).