Topping lift
To rig run a line from the boom end up through the topping lift end block and back to the boom. The running end (end that you grab on to) will need to be long enough to accommodate the full range of adjustment plus some to tie it off with. A cleat on the boom end would be handy if you see yourself reefing much.
You use the toping lift on two occasions. When the mainsail is down and when you drop the mainsail to reef it. You normally do not have to adjust it much.
Welcome aboard
For the record I didn't know what it was for either when I got started. Keep the faith it gets easier once you know the lingo.