An equilizer lets you control the low, mid, and high ends of sounds and everything in between. If you look at it like this, it might help you understand----> The left side is the low end(bass), the middle is the mids, and the right side is the high ends(treble).
The equilizer lets you adjust certain frequencies to obtain the sound you want, whether that be more bottom end(bass) more high end(treble) or literally ANYTHING else that you can think of.
Try some search engines and just surf from site to site untill you find a page that you understand, there's tons out there.
So, I hope this helps you out a bit, I've taught myself and I understand what an equilizer is and does but I've never really explained to anyone before.