What is the equilibrium?

Hint: Equilibrium is a condition of the state of a system. Every system has the tendency to remain or return back to its equilibrium state as it is the most stable position and state. The conditions for equilibrium are different for different situations but the condition which is common in all situations is that the system possesses minimum energy in equilibrium.

Complete step-by-step solution:
In physics, equilibrium is a condition of a system wherein, neither the state of motion nor the energy or internal energy of the system changes with time.
Equilibrium is a condition which every system wants to achieve as the system is most stable in its equilibrium state.
For a system to be in mechanical equilibrium, it will have zero linear acceleration and angular acceleration. When forces are acting on a system, it will be in equilibrium when the vector sum of all forces is zero. Similarly, a rigid system will be in equilibrium when the resultant torque on it will be zero.
When any object is displaced from its equilibrium position, a restoring force develops in it which tends to bring it back to its equilibrium state. Therefore, equilibrium is a condition of a system where it is in its most stable state and there is no change in its energy with time.

Note: In simple harmonic motion, the restoring force is directly proportional to the negative of the displacement. The negative sign indicates that the direction of force is opposite to the displacement. A system is in its most unstable state when it has maximum displacement from its mean position. When a body is at maximum displacement from its mean position, its potential energy is maximum.