A lead is someone who left their contact — that is, gave you the chance to reach them. They saw an ad, filled a form, or messaged you. But a phone number isn't intent yet: among leads there are people ready to buy, mere browsers, and accidental clicks.
That's why leads must be qualified. The core mistake in marketing is treating all leads as equal and making their count the main metric. What matters is quality, not quantity.
Lead stages: MQL and SQL
Leads are usually split into stages. An MQL (Marketing Qualified Lead) is a lead that meets marketing's criteria and is worth passing to sales. An SQL (Sales Qualified Lead) is one that sales has reviewed and confirmed as a real opportunity. These stages protect the sales team's time from empty calls.