Grommets are great for stuff like rope pass-throughs on sails, making and reinforcing holes for tarps and covers. You fix the ring to the material by hammering it with the right tool in between, so it attaches to the stuff you're fastening and creates a 16 mm hole for threading ropes or hooks through. This pack has 30 of these 16 mm sail grommets that you can crimp with suitable pliers.