At the end of your opponent's Movement phase, you can select one enemy unit that was set up one the battlefield within 12" of the bearer's unit during this phase; the bearer's unit can then either:
Shoot, but when resolving those attacks the bearer's unit can only target that enemy unit (and only if it is an eligible target).
Declare a charge against that unit (note that even if this charge is successful, the bearer's unit does not received any Charge bonus this turn).