CAD stands for Cover and Defuse. It's a common tactic used in S&D and Ghost Mode.
After the C4 has been placed, the remaining BL force will often camp around the bomb site and try to kill the GR as they are defusing the bomb. Therefore, it's important that the GR team goes together to the bomb site and once there, everyone should spend a few seconds to scout around for possible camping position. Then while one player (With defuse/setup kit) works on the bomb, the other should keep watching, or, in case of Ghost Mode, keep firing around the area to find hidden Ghosts around. As the timer runs low, this tactic is critical to pull off - if you can protect your teammate, he might be able to defuse the bomb on time, but if he gets killed, the others won't have enough time to take his place.