MFA stands for Made for adsense sites. A lot of these sites are buying low cost keywords in adwords and some of these ads appear on your sites. These advertisers usually pay only between $0.01 to $0.05 per click. The idea is to to eliminate those sites from showing in your adsense ads. Their business model is like "buy low (keywords), sell high".

In adsense, there is a facility called competitive filter which you can use to block off these MFA sites. Detecting them is quite simple. If you see more ads than content, then that's a very likely candidate. Another is if you see scraped content.

To jumpstart your competitive filter contents, try visiting adsblacklist.com and you can see a lot of those MFA sites. They maintain a list of known MFA websites. Just cut and paste it to your competitive ad filter in your adsense account.