For every action a new contact takes they will gain 1 rocket. This includes opting in to a funnel, watching a video, clicking a button, requesting more information etc.
Leads will lose 1 rocket for every week they are inactive - eventually going 'cold' and dropping to a lead score of 0 (no rockets).
Sort by rating
You can sort your contacts by lead rating when doing an Advanced search.
You can also manually update a contact's lead score from their profile page. Just click on the rockets to assign a score for that contact.