In email terminology, a catch-all email address is one where either one of the following conditions are met:
1) The email server accepts ANY email address as a "valid" address, regardless of whether it is valid or not
2) The email server routes ALL email addresses that don't match an existing user to a particular inbox
Although catch-all addresses aren't necessarily bad, they are a sign of a domain you should think twice about mailing.
Lets take an example of how this might come back to bite you. Lets say the domain example.com is hosted on Google Apps. Now lets say *@example.com routes to the address email@example.com. What could happen?
Someone might subscript firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, etc... to your email list. You're now sending hundreds, if not thousands of emails to the same inbox at the same domain, and guess what, you're upsetting your potential for inboxing gmail at the same time!
Email Ready makes sure you are aware of which domains are catch-all, so you can make the decision of what to do next.