PokerTracker

Could you please explain exactly what this means? I think I know but I need to be sure. Thanks:)

holdem_hand_player_detail.val_p_raise_aggressor_pos > holdem_hand_player_statistics.position


can I use this in preflop calculations?

That means that the position of the "raise aggressor" (the last player other than the active player to raise preflop) is greater than the position of the active player: i.e. further from the button. So the active player has position on the preflop aggressor.
You can use it in preflop calculations, but bear in mind that it is the last player to raise (other than the active player).