It's referring to the Multimedia Messaage System (MMS), which uses the Simple Message System (SMS) as a conduit to enable phone users to pass multimedia attachments around. Think of it like a form of e-mail attachment. The text is sent that contains information for the phone to know where to connect to download the actual file.
Where the problem lies is that Android, like many other smartphones, tries to go one step ahead of you so you don't get frustrated in waiting. They pick up the attachment ahead of time after it receives the text, sets it up for you to see, and THAT'S where the exploit lies.