A rigid body is in mechanical equilibrium when the sum of the forces, and torque, experienced by the system is zero.
Equilibria can be stable or unstable:
This animation permits the illustration of the forces involved. The position of the center of gravity (or center of mass) of the system, in relation to its support (balancing point) is the important element here.
Click on a screen to enlarge. Click on 'play' to start.