A U-track is a length of channel steel set into the ground with the open side facing up. The gate's wheels run inside the channel. The two sides of the channel keep the gate from kicking sideways, and the bottom of the channel takes the weight.
U-track has the advantage that wind, kicks, or traffic can't push the gate sideways off the rail like they could with a V-track. The disadvantage is that the channel collects every leaf, pebble, and bit of mud that washes across the driveway, so it needs more cleaning than a V-track.
U-track is less common than V-track in residential work but turns up on commercial sites with high security requirements where the gate must positively stay on its line.