On a cantilever gate, the top rollers are what the whole gate balances on. The cantilever rail rides between two pairs of rollers bolted to a heavy bracket on the post. The gate's weight pulls down on these rollers, and the tail on the far end balances it.
Top rollers usually come as a packaged kit from sliding-gate hardware suppliers, sized for the rail profile (e.g. 80x80, 100x60) and rated for a gate weight (commonly 400 to 800 kg). The rollers are nylon or polyurethane wheels on sealed steel bearings, set in a steel housing that grips both faces of the rail.
On a tracked sliding gate, the term 'top roller' or 'guide roller' refers instead to a small idler at the top of the gate that just keeps it from tipping over. Same name, different job.