Welcome.
If you ask for a simple explanation for every inagme source code line or object: There isn't :-)
You need to read and understand the code.
Check the WIKI pages (
http://wiki.esoui.com/Main_Page) and do the addon tutorials (Right side of the Wiki -> "How-To Guides" /
http://www.esoui.com/forums/showthread.php?t=6399)
Check the source code offline (
http://www.esoui.com/downloads/info1...ourcecode.html) in your editor, or online (
http://esodata.uesp.net/current/index.html /
https://github.com/esoui/esoui )
Ask questions in the forum here or the gitter channel (
https://gitter.im/esoui/esoui)