A magic ring (or magic circle) is an adjustable starting loop for crocheting in the round. You work the first round of stitches into the loop, then pull the tail tight to close the center hole completely — essential for amigurumi.
Unlike starting with a chain ring, the magic ring leaves no gap in the middle, which matters when you're crocheting a toy that can't show stuffing through a hole. It's one of the first techniques amigurumi makers learn.