IAmA (as in http://www.reddit.com/r/iama/) = I Am A

AMA = Ask Me Anything

Switch search engines :) It's the first result for searching "reddit what does AMA mean" on duckduckgo.

It's the first result on Google for that query too.

Not for me

http://i.min.us/imFTSK.png

I don't mean that exact link is the first result, I just mean a link which explains it is the first result.

UrbanDictionary is usually my first stop for decoding this kind of slang:

http://www.urbandictionary.com/define.php?term=AMA