A trailing big runs into a ball screen for the guard pushing the ball up the floor. It happens before the defense has found its matchups, usually somewhere between the three-point line and the top of the key.
Why coaches use it
Transition is where defenses are worst, and a drag makes them solve a two-man game while they are still sorting out who has whom. You are not asking the defense to guard a better action than it would in the half court. You are asking it to guard the same action a second and a half too early.
It also requires no call. A trailing big who knows to drag the ball whenever nobody stopped him is running your offense without a play. For a young team this is often the single most productive thing you can install.
Spacing does the rest. Wings who sprint all the way to the corners pull the low help away from the roll, so the drive has room even if the screen is a poor one.
How it is guarded
Everything is harder in transition. ICE is nearly impossible, because the screen happens in the middle of the floor rather than on a side. Switching is the common answer for that reason, and it is also the reason drags are often slipped or ghosted.
Most teams default to drop and accept the pull-up, with the wing defenders sprinting back to the paint first and matching up second. If the corners are filled, that drop is being asked to cover a lot of ground.
Two drags back to back is a double drag, and the problem for the defense compounds.