Multi-pass rendering

What does this mean, and how do I do it?

I have a sort of a rough idea in my head, just by knowing what the words “multi” and “pass” mean, but what can I use it for? how is it useful? and above all, how can I implement it in my OGL applications?

You are building the screen in several (multiple) passes. Blending is often used to accumulate the results. This is something bad but many algorithms needs it like shadows and other cool effects.

If you have multi texturing can the number of passes often be reduced. Download the red book from gamedev and start to read about blending and other stuff.