Music on hold is a feature which plays music to callers who have been placed on hold or are currently in a call queue.
The music can be customized to play music pertaining to the demographic base of the callers, or simply present the callers with a number of custom recorded messages relating to the potential nature of the call.
An example of this would be to record a message which can then play every n seconds indicating that the call is "important to us and will be answered in priority sequence".
Another option is to record a message which can be played to callers to inform them of a potential issue which has been made aware (e.g. "we are currently aware of the escaped monkeys and are working to apprehend them. If this call is not concerning the ravenous monkeys, then please continue holding").