to not go near something, or to prevent someone or something from going past a particular place

Barriers were built to keep back the flood water. 人们设置了障碍物来拦截洪水。

to not use the whole amount of something so that there is a small amount remaining for later

to not tell someone everything you know about a situation or an event that has happened

I suspect she's keeping something back. 我怀疑她有所隐瞒。