Concealment is a defensive trait that causes attacks to miss a certain percent of the time. So someone attacking a target with 25% concealment will miss a quarter of the time due to the concealment. This is applied independently of the attack roll; both the attack roll and the concealment roll are made for each attack, and only if the attacker succeeds at both is the attack successful. Concealment rolls are not themselves displayed on the screen, but if the roll fails, the attacker is told the attack failed because of concealment and is told the amount of concealment.
Concealment can be partially negated by the blind fighting feat, which lets an attacker make two concealment rolls for each attack, and only by failing both rolls is the attack blocked by concealment. This effectively squares concealment, causing 50% to be effectively 25%, and 10% to be effectively 1%.