Usually it means that someone on the site has gained valuable insight and knowledge from your postings and would like to acknowledge their respect in you by asking to be your friend. If you do not wish this to take place, a simple, kind and respecfull PM back to the sender explaining your decision to decline their friendship request would be valuable, as in the future, their friendship maybe valuable to you.
BTW My best friend in the forums is a person that I meet up with all the time, to help out with each others coaches, go on trips with and phone on at least a weekly bases....and yet we have never sent each other an airforums friendship resquest; but a lot of people on this site know's who this is.
Cheers
Sidekick Tony