Key Result weighting is used when calculating the Progress of an Objective from its Key Results.
Normally each Key Result has a weighting of 1 so the Objective Progress is the average of the Key Result Progresses.
You can change the weighting of Key Result to a number between 1 and 100 (we do suggest keeping to a fixed range of 1-5 or 1-10). Then the Objective Progress is weighted towards the Key Results with higher numbers - the weighted average for explanation of the calculation used.