Have you ever seen those scales/balances that work by putting weights on both sides?
If you put a one-kilo metal weight on one side and fill up the other side with grain until the balance is level/even, you have one kilo of grain.
Well, you do the same with the positives and negatives of a proposal. If the positives outweigh the negatives (they are heavier), then you proceed with the proposal.
You "weigh up" the pros and cons.
You can use the metaphor for any situation that makes sense.