What does it mean for Sabbath to be a Sign of the Covenant?

The lesson topic turns us back to a contentious comment from earlier this season - the Sabbath is the third of three things identified in the bible as "signs of a covenant". What is a sign for, how does it function, and how can Sabbath be such a sign? This episode poses more questions than answers, and your comments are welcome at sabbathschoolfromhome@gmail.com (as always).