DTIM stands for Delivery Traffic Indication Message.
A DTIM is a countdown mechanism for WLAN Access Points, informing clients of the next window for listening to broadcast and multicast messages. When the Access Point has buffered broadcast or multicast messages for associated clients, it sends the next DTIM with a DTIM Interval value. Access Point clients hear the beacons and awaken to receive the broadcast and multicast messages.
Set the beacon DTIM period:
iwpriv ath0 dtim_period x
Replace 'x' with the desired DTIM period in ms
Get the current beacon DTIM in ms:
iwpriv ath0 get_dtim_period
You'll get something like that: