The Enqueues Oracle metric are locks that serialize access to database resources. This event indicates that the session is waiting for a lock that is held by another session.
This query just shows sessions waiting for enqueue resources
SELECT sid, serial#, event, wait_class
FROM v$session WHERE event LIKE 'enq:%';
To get description of an enqueue resource use following query.
SELECT
eq_name "Enqueue",
ev.name "Enqueue Type",
eq.req_description "Description"
FROM v$enqueue_statistics eq, v$event_name ev
WHERE eq.event#=ev.event#
ORDER BY ev.name;