Flak armour consists of multiple layers of different ablative and impact absorbent materials designed primarily to deflect or absorb the majority of the force from a shot or blow. It is meant to provide defence against low-velocity, dispersed damage, such as explosions, shrapnel and ricochet material, rather than to protect against a direct impact, in which case the armour's protection is almost negligible.

Source: https://wh40k.lexicanum.com/wiki/Flak_armour