It refers to the mode of payment that is purely based on qualifying leads.

As the keyword suggests, pay per lead represents the payment mode made for online advertisements that are solely based on qualifying leads. As per an agreement an advertiser is eligible to pay only if leads are generated in their destination site. Payments are not made when visitors who turn up and yet not sign up. A lead typically consists of contact information and other general information pertaining to demographics. These leads can either be simple in having a single email id in them or a complicated one in having numerous pages. It is very important to check the leads once in while lest false ones hit you hard