A hook to level the Ender 3 (Pro) gantry. Raise Z to ~225 or so, slide the hook over the top and into the backside of the gantry. Lower it down until the gantry is forced straight on the hooks. You are now parallel with the top frame.

If you have dual-z motors, they are now level once you've heard them bind up and stop. If you are on belted dual-z, you'll have to take one of the cogs loose, force the gantry down onto the leveling hooks, and then tighten the cog into place. Tighten the belt before letting go of the gantry so it can't spring back up.

Obviously, make sure your frame is level before you do this. Otherwise you'll just level your gantry to an unlevel frame.

For any more info you can follow the TH3D guide for their dual Z kit.