Jump rings are (usually metal) rings used to make chains, jewelry and chainmaille. They are made by wrapping wire round a mandrel to make a coil and then cutting the coil with wire cutters to make individual rings. The rings can be assembled one by one into chains, earrings, objects such as bowls or ornaments, and chain mail clothing.
They can attach a clasp to a chain. Or dangles to each other. They can change the direction a charm hangs, or add a bit of length in a finishing. And if you like chain mail, you can make an entire necklace, bracelet, vest, shirt — anything, really — with jump rings.