A V-track is a length of steel bar with a V-shaped top. It is set into the ground, usually grouted into a poured concrete strip, and bolted down. The gate has grooved wheels underneath that drop into the V and roll along it.
V-tracks are simple and reliable, but they need a clean track. Leaves, gravel, sand, and pebble drainage strips are all the enemy. Wet or coastal sites also need stainless or galvanised V-track to resist rust where the rollers wear the coating.
Common residential V-track is 25x12 mm. Heavy commercial sliding gates use 40x20 mm or larger. Track length runs the full opening width plus the runout space.