If a unit elects to perform a Stand & Shoot reaction it readies its missile weapons and gives the charging unit a swift volley. A Stand and Shoot reaction can only be declared if the unit has missile weapons of some kind.
As the unit requires time to aim and fire its weapons, a Stand & Shoot reaction can only be declared if the range to the enemy is greater than the charging unit's Move characteristic.
If a Stand & Shoot charge reaction is declared, the unit makes a normal, although out of sequence, shooting attack against the charging unit (see the Shooting Phase on for more details on shooting attacks). Once the shooting attack, and anycaused by it, have been resolved, the unit is treated as having declared a hold reaction.
A Stand & Shoot reaction can even be declared against an enemy unit that starts its charge outside the firing unit's maximum range – the shooting is resolved normally assuming the enemy is just within maximum range of the shooting unit's shortest-ranged weapon. If the charge fails, for whatever reason, we assume that the chargers closed to within the weapons' maximum range before being driven off.
In most cases, there's no downside to a Stand & Shoot over simply holding. You'll probably manage to pick off a few enemies as they charge, so it's normally a good idea to shoot if you can.