Barriers are walls, either physical or metaphorical. They can block movement — the Great Wall of China was a barrier to block invading forces from entering. A window shade is a light barrier. Failing English is a barrier to getting into Harvard.
When people are being excluded from joining a clique, you could say that the clique has erected a social barrier. When fighter jets fly faster than the speed of sound, they break through what feels like a wall in the sky called the sound barrier. The Green Monster at Fenway Park keeps Boston Red Sox fans from being able to watch games from outside the stadium — it's a barrier to sight. But when line drives hit the Green Monster, bouncing the ball back into left field, the Green Monster acts also a barrier to easy home runs.