The description for the list is a short block of text that is displayed on the list's signup page. The description should give potential subscribers an idea of what kind of content you will be sending, as well as the frequency of emails they can expect.
When you first activated a list, you had the option to enter that text. But you can change it at anytime in a few simple steps.
First, navigate to the list from the Lists's page.
Then, click the Settings button, then click List Settings.
In the modal window that appears, enter your new description in the Description field.
Finally, when you're done entering text, click the Save Settings button. This will update your list's description and immediately be reflected on the list's signup page.