It means there are a lot of people in some place, very crowded. usually describe a big event, a scene.
含义：某地有很多人，非常拥挤。通常用来描述一个大的活动或者一个景点。
The word comes from Chinese idiom. It is a Chinese English word, just like "long time no see".
这个词语来自中国成语，和“long time no see”一样属于中式英语单词。
【例句】
The parade is great, there is people mountain people sea.
阅兵式非常壮观，人山人海。