a long pole, fence, wall, or natural feature, such as a mountain or sea, that stops people from going somewhere

Barriers have been erected all along the route the Pope will take. 在教皇将要经过的路上，沿途都设置了路障。

The mountains acted as a natural barrier to the spread of the disease. 群山成为了阻止该疾病扩散的天然屏障。

a gate in some railway stations through which you must go to get on a train

Passengers are requested to show their tickets at the barrier. 旅客需要在检票口出示车票。

anything that prevents people from being together or understanding each other

Despite the language barrier (= not speaking the same language), they soon became good friends. 尽管语言上存在障碍，他们还是很快就成为了好朋友。

Shyness is one of the biggest barriers to making friends (= something that makes this difficult). 害羞是交友的最大障碍之一。