Bobbin thread is a very lightweight thread. This very thin thread is most commonly used in the bobbin of a sewing machine, but it does have other uses in sewing. Although it is called bobbin thread it is not the thread that is always used in a bobbin. Most sewing will use the same thread in the upper threading and the bobbin of a sewing machine.
Bobbin thread is a lightweight thread for machine embroidery or machine basting.
When bobbin thread is used for machine embroidery, it allows the back of the embroidery design to be much less dense than the front of the design. This is especially helpful when you are embroidering lightweight fabric. It prevents the already dense embroidery from being even more dense and rigid than the fabric and keeps it more pliable than it would be with regular thread in the bobbin.
Machine basting with bobbin thread, will leave lightweight thread to be removed, and because the thread is lightweight, in many instances it can be left in rather than fighting to remove the basting.
When bobbin thread is used for machine basting, it is a more delicate thread than your regular sewing thread. It will not over power the fabric in any sense.
As with any sewing thread, quality does make a difference. A quality bobbin thread can be almost as strong as normal sewing thread. Regular weight sewing thread would be used in almost all sewing but bobbin thread is a welcome addition to your sewing basket when the need arises.
Sewing very fine fabric is one occasion that you will love working with bobbin thread. Since bobbin thread is a thinner thread, it will not be as over powering as regular weight sewing thread. A machine blind stitch sewn with bobbin thread on shear fabric can be much more invisible than if it is sewn with regular thread.
Because bobbin thread is thinner, don't sew areas of a garment or item that will rely on the strength of the seam. I for one, would never sew the crotch of even light weight pants with bobbin thread. Because a crotch seam is dependent on seam strength, bobbin thread is not the best choice. Under-stitching and hems are two common areas to use bobbin thread to prevent over bearing thread appearance.