Locking a JobCast is a way to slow the submissions on a JobCast. All agencies working on the JobCast will be notified that the JobCast is locked and the reason why you are locking it.
If you are going to be away for a period of time, if you have already identified finalists for the position, or if you just want to reduce the number of candidate submissions, locking your JobCast communicates this status to the agencies working your JobCast.
Though a JobCast is locked, agencies can still submit candidates to your JobCast. However, they understand that you may not actively review their candidates.