Moments are forces that cause an object to rotate about an axis.
Likewise holding an object in the hand causes the arm to rotate about the elbow, and the arm to straighten. In a similar fashion, when lifting, the trunk, limbs and object being lifted generate a moment about the spine. When we lift an object, the moment created at the spine is offset by larger muscle moments acting in the opposite direction. How much larger depends on how fast we accelerate the trunk, limbs and object. The muscle moments create compressive forces on the spine that can be very large in comparison to the weight being lifted.