Bobbin tension.

Bobbin tension is the drag on the lower thread as it leaves the bobbin case, and it is what the needle thread pulls against so the stitch locks inside the fabric rather than on one face. Set it before top tension, and judge it on a stitched satin column: the underside should show about a third bobbin thread down the centre, top thread either side. The lock travels toward whichever thread pulls harder, so too much bobbin tension drags it to the underside and narrows that stripe, and too little lets the needle thread haul bobbin color up onto the face. Lint packed under the tension spring shifts the setting on its own, so a head that ran clean yesterday breaks today, and every break is a stopped machine plus a garment that may already need stitches picked out.