Talk:Settings/@comment-114.77.182.116-20160909082928/@comment-27279105-20160909102829

a mouse thumb button is an additional button on some mice that sits near where you hold the mouse with your thumb. if you're in a browser, the default action of this key is to go back to the previous page.

so i guess this option means that you can use this button instead of klicking the "back" button in warframe's menus.