The % of people at one stage who move to the next stage.
Conversion rate = (conversions ÷ visitors or sessions) × 100%. Every step in your funnel has one. Homepage to pricing page: 20%. Pricing to checkout: 15%. Checkout to purchase: 60%. These multiply: 20% × 15% × 60% = 1.8% overall conversion (homepage to paid). Where the funnel drops the steepest is where to improve.
Improving conversion is often cheaper than increasing traffic. Doubling conversion = doubling customers at the same ad spend. CRO (conversion rate optimization) as a discipline exists for this reason.