A V-track is a length of steel bar with a V-shaped top profile, set into the ground (usually grouted into a poured concrete strip) and bolted down. The gate carries V-grooved bottom rollers underneath that drop into the V and run 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.