A U-track is a length of channel steel with the open side up, set into the ground so the rollers run inside the channel. The flanges of the channel keep the gate guided sideways, the bottom of the channel carries the rolling load.
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.