DFS, when used in the context of wireless networking, stands for Dynamic Frequency Selection.
DFS is a spectrum sharing technology which allows Wi-Fi to operate in 5GHz frequency bands allocated to radar systems without causing interference. In order to do this, devices that support DFS will continually monitor the frequency for potential radar signals. If a radar signal is detected, the device will leave the frequency, switch to another channel, and stay away from the channel with a radar signal for a set period of time.
DFS channels include 52, 56, 60, 64, 100, 104, 108, 112, 116, 120, 124, 128, 132, 136 and 140.