Eyelets are little round metal rings that can be pressed into your fabric.  They come in different sizes and consist of two parts:  front eyelet and back eyelet.  They can have different uses but mainly used for when you want your fastener to be a lace up.  Common garments to find them in are corsets and shoes.  It's a way to have a hole in the fabric but the eyelet provides stability and prevents this hole from tearing further.  They can also provide a unique way to embellish your projects.  This demonstration provides the necessary instruction you need so you can learn how to install eyelets in a very easy manner.