To understand this concept one must imagine one rigid body either in rest or in movement. There will be certain external forces that will manipulate that rigid body to change its current state of condition. If by force it will still remain in its formal state of condition as in with or without movement then the theory to explain this situation is called equilibrium of that rigid body.
There are certain things to remember as in the velocity that should remain constant for this theory to act in reality or more appropriately zero acceleration. Only then the results of those forces will be zero. There should not be angular acceleration too.
When you will explain this with mathematical formula, it’ll be:
R = ∑F = 0, M = ∑r x F = 0.