A moment is a reactionary torque which opposes rotation about a given point as generated by outside forces.
Consider Newton's first and third laws of motion:
- An object at rest stays at rest until acted upon by an outside force
- For every action, there is an equal and opposite reaction
If we thought about a block undergoing linear motion, a force acting on the block would cause the block to move only when the frictional force opposing the motion is exceeded by the applied force.
This idea can also be applied to rotational motion when a torque is applied to a beam. If when a torque is applied to a beam, the beam does not move, there is said to be a moment at the axis of rotation which resists the rotation.
For static systems (systems in which the components are at rest), the principle of moments states that the sum of moments within the system must be equal to zero.
Note: To help clarify this idea, if the sum of moments were not zero, then the object in question would be actively rotating.