Eq is easy to understand but very hard to learn how to do.
If you understand how a spectral analyser works, you know how eq works, it just lets you make particular parts of the spectrum louder.
As for why and when you should do, that's a bit harder.
Generally speaking, you boost the frequencies in a sound that you think are important and not loud enough, and cut all the frequencies you don't need- but it can be hard to know which bits you need, which bits you don't need and how much to boost or cut them by.
Youtube will have some good tutorials, I'm sure, if you just want to understand basic function