Most anti-spam technology is aimed at stopping spam coming in to your inbox. However, large service providers, such as Yahoo! also have to deal with preventing spam from being sent from their systems.
The system monitors the email sending trends of the user, cross-references that with the length of time the user has been using the service and when the user sends email more frequently than they have previously (or when certain thresholds are exceeded for new accounts), the user is prompted to enter a pass phrase displayed on their screen in a graphic.
Using a technique that makes it difficult for computers to "read" the numbers or letters from the graphics, the system requires that a human be present to read and then enter the pass phrase.
This technique is very similar to the method that is used by Yahoo! to verify sign-ups.