A cross brace is a diagonal piece of steel welded between two corners of a gate frame. It resists racking, which is the frame's tendency to deform from a rectangle into a parallelogram under its own weight or under wind load. The brace converts that racking force into a tensile or compressive load along its own length.
The rule is to brace from the bottom-hinge corner up to the top-latch corner. That puts the brace in tension under the gate's dead-weight droop, which is the more reliable load case for a slim member.
Cross braces are common on tall gates, double-swings with long leaves, and any gate where the infill alone (battens, slats) doesn't add enough racking stiffness. Battens fixed top and bottom give some racking resistance, but slats fixed face-only give very little, so slatted gates often need explicit bracing.