A barrier is something such as a rule, law, or policy that makes it difficult or impossible for something to happen or be achieved.

A barrier is a problem that prevents two people or groups from agreeing, communicating, or working with each other.

A barrier is something such as a fence or wall that is put in place to prevent people from moving easily from one area to another.

A barrier is an object or layer that physically prevents something from moving from one place to another.

You can refer to a particular number or amount as a barrier when you think it is significant, because it is difficult or unusual to go above it.