Cover vs Concealment
Most people don't comprehend the difference between "cover"- something that protects you from shrapnel, bullets, etc., and "concealment" - something that hides you from sight. More to the point, they think that a lot of things are "cover" when they're actually "concealment"
Several examples:
The thing is, depending on what the other guy is shooting, even a concrete wall won't provide cover for very long.