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 best way to explain this is by using an example...

If a 10N weight were to be placed on a beam, the trestles (P and Q) would have to increase their upward force by 10N


if the mass was positioned closer to Q, Q would have to exert more force than P. Similarly, if the mass was positioned closer to P, P would have to exert more force than Q.