What is a bending moment? A plain-words explanation

Short answer: a bending moment is the turning effect that a load has on a beam or truss. It is the force multiplied by the distance it acts over, and it is largest at the middle of a span carrying a load there.

A simple picture

Hold a ruler by its ends and press down in the middle. The ruler bends. The harder you press, and the farther apart your hands are, the more it bends. That bending effect is the moment.

For a single load P at the middle of a span L, the largest moment is P × L ÷ 4.

How a truss handles it

In a truss, the moment is carried by the top and bottom chords. They form a pair, one pushed and one pulled, held a distance h apart by the truss depth. The force in each is about the moment divided by the depth:

chord force ≈ (P × L ÷ 4) ÷ h

That is why a deeper truss has smaller chord forces, and why a longer span has larger ones.

Where it is biggest

At the middle of the span, under the load. The moment falls to zero at the supports. So the chords near the middle carry the most, and near the ends the least. That is why you can use lighter members near the supports. See where to cut weight.

This is a simplified picture, good for comparing designs. See calculating forces.