If the support is not directly under a mass, it will not balance. A moment is created by the offset between the support and the center of mass. The see-saw is in trim. The see-saw is NOT in trim. When calculating moments for the seesaw, we used the distance between the force and the pivot point as the moment arm.
Stability is the tendency of a system to return to an equilibrium state after it is perturbed.
It is commonly described by talking about a ball in a valley, on a plain or on a hill. Adding heavy blocks to the top of the see-saw shifts the center of mass above the pivot. When the left end of the see-saw is shifted down, the center of mass moves to the left, and a moment is produced that rotates the left end further down. Adding heavy blocks to the bottom of the see-saw shifts the center of mass below the pivot. When the left end of the see-saw is shifted down, the center of mass moves to the right, and a moment is produced that rotates the left end back up.