Welcome aboard Aloha28!
It's known as a "jack line" used to allow the sail to fall away from the
mast when you pull it down to reef at the first cringle. This way all the cars or slugs don't make the sail all bunched up under the cringle and you can easily and neatly roll up the sail along the foot. (And/or you don't have to remove cars/slugs from track/slot when reefing.) It can be confusing because there are other lines on a
boat that are also be called jack lines, like the ones you clip a tether to. Sorry I can't attach it but if you
google "mainsail jackline" you'll see a bunch of stuff about them.
By the way, the main should be hoisted higher so the luff is taught with no wrinkles in that sailing shot.
Edit, on second look at your photos I see you don't have a stopper in your slot so the slugs will fall out when you reef. That might be a nice thing to get next time you go to the
chandlery. Nothing wrong with letting it fall out, it's just a hassle to slide them back in when it's time to hoist the main back up.