A ledged-and-braced timber gate hangs entirely on its ledges, so without a diagonal brace the leaf's free (latch) edge droops under its own weight within a season. The brace is one board, cut from the same section as the ledges, running from the bottom hinge corner diagonally up to the top latch corner. That direction matters: run the wrong way, the brace does nothing to stop sag, and can actually make it worse.
The direction the timber brace runs sets how it carries load, which is the opposite of a welded steel cross brace. A slim metal cross brace works in tension, it is a thin bar that only resists being stretched, so it is fitted bottom-hinge to top-latch to be pulled taut as the leaf tries to droop. A solid timber diagonal brace is thick enough to push back as well as pull, so fitted the same way, bottom-hinge to top-latch, it works in compression: the leaf's own weight tries to close the diagonal up, and the brace resists by pushing outward along its length.
On a double-swing gate each leaf gets its own brace, and the two mirror each other so the closed gate reads as a symmetric V or chevron rather than two parallel diagonals running the same way. CAD60's timber swing gates default the brace on for exactly this reason, an unbraced ledged gate sags on its latch edge inside a season, and a brace running the other way just pulls its own fixings out.