could someone explain me what this means?
Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_2_1 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8C148 Safari/6533.18.5)
That is an iPhone Safari mobile user agent string. iPhones can be configured to add this string whenever somebody posts, although I don't know why anybody would want it displayed. From left to right, it basically says: "this web browser is compatible with Mozilla 5.0, runs on an iPhone with strong security using iOS 4.2.1, which is based on Mac OS X using American English. I use the Apple WebKit rendering engine, version 533.17.9 based on KHTML, an open source HTML layout engine similar to Gecko. This is version 5.0.2 running on iPhone firmware version 8C148. The build number of this browser is 6533.18.5."
If you want to analyse other user agents, you can go here.