What is the big idea of mechanical equilibrium

The body won't move or it will be moving with constant velocity

Explanation:

A body is said to be in mechanical equilibrium if the total force acting upon it is zero and the total torque acting on it is zero.[1] As the net force and torque on a body are zero, it will continue to move in a straight line with constant velocity and rotate with a constant angular velocity. If the velocity of a body in mechanical equilibrium is zero, then it will remain at rest.

This can be easily understood by interpreting the third law of Newton, the statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.