Equilibrium

Newton's first law states that;

a body will remain in  a state of rest or unaccelerated motion unless acted upon by an external resultant force.

This basically says that if there is no resultant force on an object the it is either stationary or moving at a steady speed. It is in equilibrium. If the resultant force on the object is not zero then it will accelerate.

If we know that an object is in equilibrium we know that there is no resultant force on it, i.e. whatever forces do act on it balance. We use this technique in Physics very often to calculate the magnitude and direction of unknown forces.