What does it really means to a condition of equilibrium i. e., if the body rotates about itself, without moving?

Explanation:

It means that there is a single resultant couple acting on it with no resultant force.

If a body is pivoted at some point, then when we apply a couple on the body then the body will rotate along that point, without moving but,  the pivot point will anchor the body and will resist the movement of the body.

 Additional Information

A body is rotating at its position without moving:

• In this case, the body is not moving from its position so its velocity is zero.
• Therefore the linear acceleration of the body will be zero and the body is said to be in translational equilibrium.
• A rigid body is said to be in mechanical equilibrium if both its linear momentum and angular momentum are not changing with time.