It means that a session is waiting to use or change an object definition or to use an SQL statement that another session is loading, changing or parsing, or is waiting to change or parse. If this event accumulates a lot of time, then it may indicate that the shared pool is too small or is being flushed regularly. Otherwise it indicates that database object definitions are being changed regularly. There is a problem with the null-refresh optimization for snapshots introduced at 8i that can cause this.

I've detected this wait event on one of my production databases.

Event Name                     Timed waited
library cache lock             615.00

I'd like to know what's it about.