Eyelets are used to reinforce holes in leather, clothing, shoes, and other fabrics. They can be made of metal or plastic, and are easily used in common projects, requiring only the eyelet itself, a eyelet-setting tool (a metal rod with a convex tip which usually comes with the Eyelets when you buy them in a store), and a hammer. Fancier eyelet presses (as shown in the picture) exist but generally a hammer and the eyelet-setting tool works at least as well for setting just a few eyelets.
Their most common usage is to strengthen the holes for bootlacing but are also good for making corsets and other laced clothing, plus curtains and other household items that require hanging from hooks. The eyelet prevents the cord from tearing through the hole, thereby providing structural integrity. Small eyelets are also called grommets, especially when used in clothing or crafting. When using eyelets for crafting, they are generally used in a decorative fashion.