what do I loose when I reset PRAM?

I just had an inability to start my MacBook Air 11." I followed instructions from one of these posts and got it going again via resetting the PRAM. I found another thread that told me what PRAM is. However, I don't understand what I loose when I reset it. Can it change anything that I needed? Do I need to back up and files from Time Machine?

Thanks.

Geoff