Greylisting is the process of temporarily deferring email delivery in an effort to reduce spam. The SMTP protocol specifies that emails can be deferred temporarily and delivery should be retried. Many spam senders aren't compliant with the protocol, and if they do not keep track of their message transfer attempts, their emails will not get delivered, and the greylisting is successful.
Greylisting does have the negative side effect of causing some legitimate email from getting immediately delivered.