The metaphor of a funnel is used because the number of customers decreases at each step of the process. When using this metaphor you can target specific groups: "upper funnel," "middle funnel," "lower funnel" for potential customers.

Typically you will have a large number potential customers on the first page, each step after that will "funnel" out potential customers. The more steps it takes to complete the task, generally the larger the loss of customers.

Shopping Conversion Funnel