I think they call this an eccentric screw adjustment. You loosen the screw, turn the washer with the off set hole, and it tightens the belt by moving the motor further away from the hand wheel.
In this case, the belt was streached too far and beyond the adjustment of this machine. I had to put a new belt on anyways. I ended up using a streach belt. They work fine but don't last as long as the original V-belts.
What a great, simple, early zigzag, sewing machine.
Love that color? Classic 50's! and it sews great.