What is the purpose of a bobbin in a sewing machine?

A bobbin winder is the part of a sewing machine that takes the thread from a spool of thread and winds it on the bobbin. The sewing machine bobbin is a mini-spool of thread, made of plastic or metal, that holds the thread that will be on the underneath portion of your fabric when you sew.

The top thread comes from the spool of thread and the thread on the bottom side of your fabric comes from the bobbin. Instead of winding the thread on the bobbin by hand, long and tedious, the machine will wind it for you if you load it up correctly. Check your owner's manual for instructions. If you don't have a manual, do a google search for it by entering the name of your machine branch and the model # and it will probably find one. Also, any experienced sewer can show you how.