Superior Threads teaches you all about sewing machine tension.
Think of the top and bottom thread as having a tug of war. If the threads are identical and you are sewing on a single layer of fabric, both sides have equal strength and the result will be a draw. The sewing should therefore produce perfectly even stitches with no top thread showing underneath and no bobbin thread showing on top. However, in the real world of sewing and quilting, the teams are rarely equal. Read what Superior Threads has to say.