A Lead (also called a prospect) is a person or organization that has not done business with you before. It could be just a person who you might have met at a tradeshow or conference. It could be someone on a list you purchased. You will need to retrieve more information regarding this lead in order to create (qualify) an opportunity in your sales pipeline.
If it is likely to become a customer, the prospect is then converted to an Opportunity, which includes information about the Customer and the Contact.
In terms of a process, a Lead goes through a verification step and then becomes an Opportunity which includes qualification steps and in turn tracks everything through to a closed deal.
For most, lead verification is simply making sure the lead responds to email or answers the phone - this is the way Odoo can be best used (since qualification is better managed via an Opportunity).