The META description button switch activates a button on the "submit link" page next to the url box which allows submitters to click to have their site title and description automatically filled in for them. It will grab the meta tag description along with the title tag from the url which they've typed. They'll still have a chance to review before submitting.

If you have keywords in a field named "keywords" then this button will also fill that field in with the value of the site's meta keywords tag.