HEAT warheads (predominant RPG warhead type) work on the principle of extremely high velocity molten metal burning it's way through armour after the warhead detonates against said armour.
An extremely fast stream of this liquid, if the warhead is of sufficient power to breach the armour, eventually burns it way into the interior of the vehicle splashing all inside with molten metal and spall (little bits of metal flying around at high speed) and usually setting fire to said vehicle and of course, the occupants. A horrible way to die needless to say...
BAR armour works on the principle of detonating the warhead away from the main armour, meaning that the molten metal's kinetic energy is already mostly expended before it reaches the main armour of the vehicle. Such armour then being capable of withstanding the molten stream.
BAR armour should also have signicant utility against HESH (high explosive squash head) warheads, given the gap between the BAR system and the main armour. This warhead type produces a massive shockwave that travels through the armour, when the warhead detonates. The benefit of detonating the warhead as far away from the vehicle as possible should be obvious...
This results in metal being broken off on the inside of the vehicle and pinging around inside the vehicle at extremely high velocity. Another very unpleasant experience for the crew inside said vehicle.
Another part of the armour protection for vehicles these days are called spall liners. These are basically thick Kevlar sheets designed to capture or prevent spall from flying around inside the vehicle...
