I would suggest you start reading Concepts Guide from docs.oracle.com
its too complex to understand
If it's too complicated then what will you say about MOS documents that are much more complicated some times?!
We would like to help you but first tell us about your background and things you do.
Start with EBS documentation -- http://docs.oracle.com/en/applications/?tab=3
We have discussed "How to become an Apps DBA" topic many times in the forum before, please see https://community.oracle.com/search.jspa?q=how+to+become+apps+dba for details.
if you are very fresher then you can take some training so it will be easy for you.