Defining the support polygon
The support polygon is the region on the ground enclosed by a robot's current ground contact points. For a robot standing on two feet, it's roughly the area between and slightly around both feet, the convex shape you'd get by stretching a rubber band around the contact points. For a quadruped standing on all four feet, it's a larger, more forgiving quadrilateral. The shape and size of this polygon changes constantly as feet lift and land during motion.
For a robot to balance the way a stack of blocks balances, its center of mass, projected straight down onto the ground, generally needs to stay within that support polygon. If the projection drifts outside the polygon's edge, gravity creates a rotational torque around that edge with nothing on the other side to resist it, and the robot tips over that edge unless something intervenes.