Hi Guy
Your description indeed fits that of a reefing hook. This hook engages a grommet on the luff of your mainsail, somewhere above the tack grommet. You should really practice using the reefing hook and the entire reefing system before you go sailing. Time spent familiarizing your crew with this gear will pay big dividends when the wind reaches a state that you need to reef.
The procedure for using the hook goes something like this:
Decide to reef the first time you think maybe you should reef
Head the boat high into the wind, barely making way, to bring the boom aboard. If you have a topping lift, connect it to the boom end. If you have an adjustable outhaul on the clew, it may pay to loosen it. Loosen the main halyard and pull down your sail in the mast track until you can hook the grommet or first reef point on the reefing hook. Take up the slack in the halyard to retighten the luff of the sail. Take up the slack in the outhaul at the reefing point on the clew of the sail. Tie off the reefing lines around the sail above the boom. Sail on.
You should practice this and have the necessary lines pre-installed. Shaking out a reef is the reverse, and you must make sure you release the reefing lines that are around the sail above the boom and release the reefing grommet at the clew. Then haul the halyard to the desired tension and you are on your way.
Refinements: it is nice to have a way to prevent the unused portion of the mainsail from spilling out of the mast track when you pull down the sail. Some of your lines may be conducive to jam cleats for faster attachment/adjustment of the reefing lines, especially at the boom end. Remember that the reefing lines that go around the sail (pre-installed, right?) are not loaded in any way and only gather the reefed sail to make it more neat.
There is more, but you will have to determine the needs of your own boat. By the way, I assumed that your mainsail has at least one reefing point, a series of large and small grommets in a line parallel to the boom about a 1/3 way up the sail when hoisted. Some mainsails come with no reef points.....