Pay Per Lead (PPL) is when you only have to pay when a customer shows they are a lead.
A lead is classed as someone showing interest and engaging with the company. For example, if you had 2000 people see your ad, 100 clicked on it, and 10 people filled in their email for the monthly newsletter. You would only have to pay for those 10 people. That is because the fact they filled in their email address it shows they are interested.
Different things that can be classed as leads are calls, emails, filling out forms or any other format where they are communicating with the business.