VERP stands for Variable Envelope Return Path. It is a technique used to more accurately detect and remove undeliverable email addresses. Vuture uses it as an automated way of determining which bounce message relates to which email address. 

It works by making the Return Path email address unique for each subscriber who receives an email. VERP allows greater control with bounce processing because the exact subscriber's email address, which created the bounce, is always included in the bounce message through the return-path address. Without VERP, it is not always possible to tell which email address bounced, because not all mailbox providers include the email address in the bounce message.

Further reading

http://en.wikipedia.org/wiki/Variable_envelope_return_path