What does the tension control on a sewing machine do?

The tension control on a sewing machine determines how tight or loose the thread in each individual stitch is. The top and bobbin thread tension is like a tug-of-war. If each side is pulling equally, you have perfect tension. Imperfect tension is evidenced by thread loops on the bottom if the top tension is too loose and the same on top if the bobbin tension is too loose.