If your unit is in combat: Pick an enemy unit your unit is in combat with to be the target of the pile-in move. Each model in your unit can move up to 3". That move can pass through the combat ranges of any enemy units, but each model must end that move no further from the target unit. At the end of the move, your unit must still be in combat with all units that it was in combat with at the start of the move.
If your unit is not in combat: Each model in your unit can move 3" in any direction. That move can pass through and end within the combat ranges of any enemy units.
Pile in: move up to 3".If in combat, the unit must end the move closer, or at least as close, to the target enemy unit.