These 1/8" (3.1cm?) ratchets have metal gears and will hold up to 34kg each (68kg between the two), which is plenty.
You attach the pulley block onto your railing or ceiling, and then raise the carabiner end by pulling down on the string. You lower the pulley by pressing the release hook on the block, and pulling down on the carabiner.
It's nice to have, if you need to change the height of your light, as plants grow.