The footer is the very bottom bit of your email showing your copyright and company information. It sits right below any Social Links you've added to your account. The footer is required, so your emails will comply with CAN-SPAM, and other laws.
The default details in the footer are the company name and address that you entered when you registered your account. You can edit this info by clicking Settings, at the top of the screen. The Company/Group, and Physical address fields show in the footer. Feel free to add a phone number, or any other details you'd like to display in the email footer, to those fields.
Activate the Campaign Tweaks Add-on to make more display changes to your email footer. You can add extra content below, adjust the copyright date display, and center the footer text.