Setout is marking where every post, gate, and panel will sit, copied from the drawing onto the real ground with string lines, paint marks, and pegs. You do it after the site is checked and before any digging.
A good setout catches errors that the drawing didn't expect: a tree root in the post hole, a buried sewer line, a kerb that's not actually parallel to what the survey said it was, a slope steeper than the contour map suggested. Catching these at setout costs an hour. Catching them after the concrete is poured costs a day.
The minimum setout for a swing gate is two post centres marked with paint dots on the ground, plus a string line between them at the correct gate height. For a sliding gate, add the runout end and the catcher post position.