What exactly Does Pre-approved mean?

I am always amazed at the number of credit card offers I get in the mail. I get a lot of them in email too. Cards offering 0 balance transfer. Maybe I should take them up on that. I could free up my other credit card. Have you noticed though that they all say pre-approved, or approval gauranteed or something like that but they're really not? I guess you're just pre-approved to apply for the loan. Really though I have good enough credit. My problem is I'm self-employeed. It seems they're not too crazy about that unless you have massive six-figure incomes or along those lines. I guess that should tell you I do not.