Swivels can provide a point between the main fishing line and the terminal tackle, and they are there to reduce the opportunity for your line to twist. Swivels are also used to attach multiple pieces of tackle to a main line, such as connecting both a leader and hook and another separate line with a sinker. It is most common to find swivels made of a nickel-plated brass with snaps and rings made from stainless steel.
Use a swivel that is properly sized for the fishing line you are using and the type of fish you plan to catch and use the best possible swivel you can afford.