When locking issue is happening, the lock holding sessions might block other sessions, and consequently, blcok more other session. The blocking tree gives you a clear view of the blocking issue. And then you can choose to kill the root session to solve the blocking issue immediately or investigate the blocking session to avoid to happen again.
Click "Block Tree" button or presss F8, you will get the blocking tree.
Select a session from this tree, then you can kill it or investigate the history details to find out the root cause. Further information of session History, please refert to "Show Session List".