Concealment

From Talus Wiki

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 Improved Invisibility) will be missed half the time (when they normally would be hit).

It can be partially negated by the Blind Fighting 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.

See Invisibility and True Seeing do not penetrate concealment.