Sorry to ask such a stupid-sounding question.... What are prefolds? And how do they work, where do you buy them etc?

I used flat terry towelling with my kids (had never heard of MCNs then, and wouldn't have had the money anyway! One of my besties sells BBH so I thought this time I'd support her business and buy a few AIOs, but size med as my kids tend to be born biggish, then don't grow a lot! But there will still be a few month before they fit and I was thinking a couple of fitted bbhs, and normal terry but prefolds sound like a popular choice.

Please fill me in!!!!