The top rail closes off the top of a gate frame and gives the upper batten or slat fixings a steel substrate to bolt or weld into. It usually runs the same RHS profile as the stiles, mitre-jointed at each corner so the frame reads as one continuous outline.
On battened or slatted gates the top rail is often hidden behind the infill. On open-look gates (loop tops, spear tops, picket-style runs) the top rail is part of the visible composition and often gets a heavier or differently-profiled section to feature it.
For pool-compliant gates, anything fixed to the top of the rail (decorative ball caps, finials, fittings) needs to fall outside the AS1926.1 non-climbable zone or be eliminated entirely so a kid can't grab and climb.