A bottom roller is a steel or nylon wheel housed in a steel bracket that bolts or welds to the underside of a sliding gate's bottom rail. The roller profile matches the track: V-grooved for V-track, flat with flanges for U-track. Each gate runs two bottom rollers as standard, one near each end of the gate panel.
Residential bottom rollers come in 75 to 120 mm diameter. Heavier commercial gates step up to 150 to 200 mm. The bigger the roller, the more easily it crosses dirt, sticks, and minor track obstructions without binding.
Bearings are a wear point. Sealed ball bearings in good rollers last 5 to 10 years before they want replacement. Cheap rollers with bushings will go in 2 to 3.