To hold spools of thread and sometimes other materials. You’ll eventually want two.
Griffin Standdard Bobbin, F1469
AET Standard Bobbin, F1472
If you’re buying two, we recommend the second be AET Flared, F1471. Though not as precise as a standard, you can apply thread, or other spooled materials such as floss, lead, wire, braid, etc.
For large bass and saltwater flies, many tyers use "saltwater" bobbins. F1468 for longer reach. For exceptionally small flies, midge bobbins are favored. F1465