It means “function” or “effect”.For example,眼睛的作用是看（东西），eyes are to see things，耳朵的作用是听，Ears are to listen。Or ，药起作用了吗？Does the drug work？这个工具的作用是。。。 ：The tool is use for。。。 Anyway I think the best way is to interprete it as the noun “function”。Hope this helps。

It can be used as a noun,or a verb,The usage of which depends on its context.

Agree with Weiyang!