e.g. 

%dbaag          ALL = /bin/su - oracle

In your system there exists a group named 'dbaag" and a user id "oracle". 
Above statement says - allow any users that are member of group "dbaag" to switch to user "oracle" using command sudo su - oracle 

This enables the user run applications/commands that were only allowed for 'oracle' user (e.g. database realted).

For detail- check linux manual or some good tutorial.