here is a very nice page about secure frames
http://www.wowwiki.com/Secure_Frames_Overview
I recommand you to buy the "World of Warcraft programming" book by cladhaire kaelten, there is a nice chapter of around 20 pages explaining secure frames and all needed for writing simple or strong addons, dealing with secure frames is not that simple because once you use such frames they are restricted in combat and you will have to deal with lots of workarounds.