A tandem-charge is a weapon that has two stages of detonation. It is effective against reactive armour. The first detonation triggers the reactive armour which disrupts the first detonation. However, after the reactive armour has done its job the same spot is more vulnerable. The reactive armour on that spot is no longer "reactive" as it is a one-time solution. The second detonation from the same projectile (which defines it as a tandem-charge) attacks the same spot as the first detonation. However this time around the reactive armour has already been used to protect the vehicle (mostly tanks) against the first detonation. So now only the regular armour plating is protecting the vehicle on that spot.