What does "Purge" do?

Hello,

   I've been using After Effects for a tiny while, I've doe a few simple things in it. I've also made my intro for my videos, which turned out well, buut one thing I can not find  the answer to! ANYWHERE!

I've been told to "make sure you purge your stuff", and I've heard it has to do with the RAM Preview option. But what does "Purging" actually do?

     Thank you for answering my question!

Thanks,