The spool holding a sewing machine's bottom thread. While any purchased spool may be used for the top thread, before stitching, thread must first be wound onto the bobbin. Most machines have a method to automate this process somewhat.
The bobbin fits into a bobbin casing before being inserted into the machine.
Sewing Machines: Bernina 530
The bobbin fits into a bobbin casing before being inserted into the machine.
Sewing Machines: Bernina 530