What does "test function" mean?

I am trying to learn weak derivatives. In that, we call $\mathbb{C}^{\infty}_{c}$ functions as test functions and we use these functions in weak derivatives. I want to understand why these are called test functions and why the functions with these properties are needed. I have some idea about these but couldn't understand them properly. Also, I'll be happy if any one can suggest some good referenc