A tack weld is a small spot or bead of weld, typically 5 to 10 mm long, used to hold two parts in their relative position before the structural welds go on. They're meant to be light enough to break or grind off if the alignment turns out wrong.
Fab-shop practice: tack the gate frame at the corners first, check it for square, then lay the full mitre welds. Tack the hinge plates to the stile, hang the gate to confirm plumb, then full-weld. Tack the cantilever rail to the gate panel, slide it on the rollers, then full-weld.
Tack-and-check is faster than welding and grinding off twice. Skipping the tack stage and going straight to full welds is how you end up with a gate that doesn't sit square and can't be fixed without major rework.