How to fit a pendant light hook?
Normally, the junction box for a ceiling light fixture is attached to the ceiling joists in such a way as to distribute the load of the light on those joists instead of on the drywall. The normal way is to hang a 2x4 between two joists, suspended a couple inches above the bottom of the joists, and attach the J-box to that so it'll be flush with the drywall. Such a structure can support hundreds of pounds, even if the j-box itself won't.

Under about 30 pounds, which includes most modern round lights, small hanging lights and ceiling fans, you should be able to mount directly to the J-box; it will have threaded holes for a machine screw. Over that, most kits will include a threaded J-hook that is designed to screw into the wood strut above the J-box; these can support up to a couple hundred pounds, and the J-box then only supports the weight of a crown cover to conceal the mount and wires.

Be wary; there are "old work" ceiling J-boxes designed to be inserted into the ceiling and have their load distributed by the drywall. These will only take about 5-6 pounds (your average round light), but nothing more. Hanging a ceiling fan or heavy hanging light directly from a J-box is a sure-fire way to end up needing to re-drywall your ceiling (after putting in a proper J-box of course).