A:
The family of snap swivels of which there are many designs are to be used to connect the end of your line (terminal end) to a lure or other fishing device to allow for quick changing of lure's and to stop line twist.
You tie the end (terminal) of your line to the split ring on the snap swivel using one of many different fishing knot's; but the "palomar" knot is one of the better knot's for this purpose.
Open the snap swivel and attach your lure or other device (not for attaching a plain hook). Use the palomar knot to tie on a plain hook or jig, not a snap swivel.
Make sure you close the snap!