Is bobbin thread different than top thread?
The bobbin thread can be a lighter weight than the top thread and still provide sufficient strength without adding bulk. Using a cotton top thread with a poly bobbin thread is fine.
What is the point of a bobbin thread?
In general, the bobbin is the thing that feeds the thread to stitch from the lower part of the machine. Its purpose is to hold the thread below the needle, and it is where the thread in which you stitch comes from.
A: Looping on the underside, or back of the fabric, means the top tension is too loose compared to the bobbin tension, so the bobbin thread is pulling too much top thread underneath. By tightening the top tension, the loops will stop, but the added tension may cause breakage, especially with sensitive threads.