An opportunity can be created if a potential lead is promising. It allows detailed tracking of the lead by adding expected sales dates, revenues and sales probability.
The goal is to make this lead a new customer. From an opportunity you can generate a quote to agree on the work that will be delivered. You can start setting up a project from an opportunity. As long as the opportunity is not won (the quote is not signed), the project will have the status 'Pending' and will show light blue in planning overviews.