Do not disturb mode is when only calls from the Queue(s) come to the Agent.
Turned on and off with the switch (Fig.1):
...
- the switch is in the on position, the Agent receives calls only from the Queue(s);
- the switch is in the off position, and the Agent receives all calls from Queues and other Users.
Fig. 1. DnD
If an Agent , or system User , has the Do Not Disturb mode enabled, such an Agent will be displayed with an 
indicator 
indicator in the list of Users (Fig.2).
Fig. 2. Status display in the User list
When an Agent turns Do Not Disturb mode on, he cannot accept internal calls from other system Users. Only calls from Queues will be routed to him.