Jump rings are round (or oval) rings that you use to attach items to each other, like a clasp to a necklace. They have many, many uses. Jumprings come in "open" - with a split so that you can open them, "closed" - permanently soldered shut, and split rings, which are like the rings that you slide your keys onto.
To open a jump ring, use two pairs of pliers. Grasp it on each side of the "break" in the ring - the opening - and twist to the side. When closing, reverse and close the same way. Pulling open the ring deforms the shape and weakens the metal.