1.28 Understand that the upward forces on a light beam, supported at its ends, vary with the position of a heavy object placed on the beam

The upwards forces on a beam, supported at both ends, varies with the position of the object on it. (Yes, I am fully aware that I just re-worded the specification point.)
Basically, if you have a plank/bridge/thing supported by 'pivots' at both ends and you then place a mass on the plank, the pivots would have to exert a greater force to equal the downward force produced by the mass

I drew this diagram to help explain the point. If the tree was closer to pivot A, it would exert a greater downwards force on it, so it [pivot A] would have to exert a greater upwards force to balance this.