"Jump rings" refer to a ring that can open using pliers.

They are available in many sizes and gauges of wire. A higher number is a thinner gauge (ie: 24 gauge is thinner than 20 gauge).

The size (ie. 5mm) refers to the outside of the ring. If the gauge is thicker, there will be less space inside the ring.

Jump rings are an essential finding allowing you to connect 2 closed pieces together, such as a chain and a clasp.