How does weightage of a KR/alignment impact the objective progress?

Weightage value is a relative measurement of impact any KR or objective alignment is going to have on the overall objective progress. Thus if there is only one KR with weightage value 10, it is going to have the same impact even if its weightage is updated to anything more than 0.

0 is a special case of weightage - when set, it doesn't impact the objective progress at all. 

Formula for calculating the impact is same as that of weighted average. For example, if there are 4 key results to an objective and and no alignments with following weightage values for KRs - 3, 5, 7, 10. When the first KR with weightage 3 is 100% complete it will impact parent objective progress by = 3*100/(3+5+7+10) = 12% whereas the fourth KR with 10 weightage will impact by = 10*100/(3+5+7+10) = 40%.

Same logic is followed for alignments weightage. For example there are 4 KR with above KR weightages & 2 objective alignments. When the first objective with weightage 5  assuming it is 100% complete, it will impact parent objective progress by = 5*100/(3+5+7+10+5+4) = 14.70% whereas the 2nd objective alignment with 4  weightage will impact by = 4*100/(3+5+7+10=5+4) = 11.76%.