A lead is an early-stage sales opportunity: someone who has filled in a form, rung your office, replied to a campaign or been introduced by a contact. They are not yet a customer, and not every lead is a good fit.
How leads are handled decides how many turn into work. Recording each one with its source, replying promptly and following up on a schedule prevents enquiries slipping through the gaps in a busy inbox. A clear route from lead to quote to order also shows you which sources of enquiries are worth your time.