From the Model Mayhem FAQ:

What are Lists? How do I make a List? What Lists am I on? I have other questions about Lists!
A list is basically a way to favorite images you see in the portfolios of Model Mayhem members. Everyone can make at least one, but subscribers get to make more lists than basic members.

Lists are easy to make – when you are looking at an image you want to favorite, just click “Add to list” on the image page.

This should take you to a page where you can confirm you want to add the image to a list. If you do not have a list created, this page should give you a chance to create your list!

For more information, check out this user guide to lists: