So, today is quickie one, just to make the links. Seems this message from OEM is not clear enough for some people, specially regarding non-specialists in Oracle: This means something is in lock in your database!
If this is the case, contact a DBA.
If you ARE a DBA, you may want to read this post about easy locating and solving locks: Solving Simple Locks Through @lock2s and @killlocker.
Also, if the session if from DBLink, is always useful to read this: Lock by DBLink – How to locate the remote session?
There is also some additional/specific material about some issues and bugs in this regard here: Tag: LOCK.
I hope it helps!