He tells Abraham to look at the stars, the stars in the night sky which are beyond numbering. The stars are beyond counting, and so will your descendants be. Then Abraham understands, and we read that Abraham believed, and God accounted it to him for righteousness. The sign had spoken to Abraham's mind, aided his understanding and clarified God's promise. And Abraham responded with faith.
Baptism is a sign which clarifies.
It tells the Christian that Jesus washes away sin just as water washes away dirt. It declares that the Holy Spirit descends upon the soul to give life just as the spring rains fall down upon the soil to give life. And the Lord's Supper is a sign which clafiries. It tells the Christian that just as bread is broken and the grape is bled that the body might be nourished and sustained, so the Lord has bled and His body has been broken so that the Christian might be given spiritual life and nourishment and growth. A sign clarifies, a seal assures.