What is a "bobbin"?

A bobbin is a small cylinder or spool of thread, often made of plastic or metal, that fits inside the shuttle or bobbin case of a sewing machine. The bobbin is used to hold the lower thread that interlocks with the upper thread from the needle, forming a stitch. Bobbins come in different sizes and are often specific to the brand and model of the sewing machine. When the bobbin thread runs out, it needs to be replaced with a new one. Some sewing machines have a top-loading bobbin, while others have a front-loading bobbin or a side-loading bobbin.