Here you can customize the opt-out page in case you have chosen the double opt-out as unsubscription mode (To change this setting, go to Preferences > Properties > Unsubscription).
Use the editor to customize the page and make sure you use the 
button to get the unsubscription link working.
Please find more information about the different unsubscription methods here.
For a complete overview of the built-in editor, visit this page.