The higher the weight is set for a specific keyword, the more often it will be used for the Super-URL instead of one of the other keywords within the same slug.
The set weight is therefore only of importance when multiple keywords are being pushed within one Slug.
Keyword A has a weight of 2. Keyword B is given a weight of 1.
Keyword A will therefore be used for the URL in 66% of all clicks, ie. for 2 of 3 customers clicking on the URL.