I guess this basically means this will make sure that their e-mails aren't caught by AOL's or Yahoo's spam filters.
The Internet companies say this will help them identify legitimate mail and cut down on junk e-mail, identity-theft scams and other scourges of users of their services. The two companies also stand to earn millions of dollars a year from the system if it is widely adopted.More details over here. According to AOL and Yahoo the system is a way to restore some order to e-mail.
AOL and Yahoo will still accept e-mail from senders who have not paid, but the paid messages will be given special treatment. On AOL, for example, they will go straight to users' main mailboxes and will not have to pass the gauntlet of spam filters that could divert them to a special bulk e-mail box or strip them of images and Web links.