Simply put. Cover stops bullets, Concealment doesn’t. Concealment..well…”conceals” you. The opponent may be able to hit you “through” what you are hiding behind but he doesn’t know you are there. Or perhaps he knows you are there “somewhere” but not sure of your exact location.
Cover on the other hand…the bad guy may know exactly where you are, he just cant hit you through it. The best of both worlds is when you are behind good cover AND the opponent doesn’t know where you are.