Concealment is a chance that your opponent will miss you when he would normally hit otherwise. For example, someone with 50% concealment (from something such as Shadow Shield) will be missed half the time (when they normally would be hit).
It can be partially negated by the Blind Fight feat, which lets you re-roll for a second chance when failing a concealment check. For example, if your first roll results in a miss due to concealment, you can roll again. So a 50% concealment would effectively become 25% against an opponent with Blind Fighting.
- Other than taking the feat Self Concealment multiple times, concealment bonuses does not stack. Only the highest concealment value is used.