NOTE: I am slightly adjusting the wording of the answer to this question by removing the word "seals," which has not historically been used by Baptists in reference to the ordinances.
Question 43: What are the sacraments or ordinances?
Answer: The sacraments or ordinances given by God and instituted by Christ, namely baptism and the Lord's Supper, are visible signs that we are bound together as a community of faith by his death and resurrection. By our use of them the Holy Spirit more fully declares the promises of the gospel to us.
The last line of the catechism answer speaks of the Holy Spirit's ministry through the ordinances of more fully declaring the promises of the gospel to us. In one sense, of course, the gospel is fully declared when it is preached to us. However, while preaching the gospel includes within it a call to faith, that faith remains in the hearer and invisible act. I may hear the gospel and either come to believe it, or if I already believe, I may continue believing it as the preaching renews my faith to face another week of life. But these faith responses remain hidden in my own heart unless I have some way to act upon them.
It is interesting that evangelicals have developed several different non-biblical methods of making faith into a public action: raising your hand, walking down an aisle, praying the sinner's prayer. None of these actions are necessarily bad, but neither are they biblical ordinances given to us by Christ to take our faith public. Baptism and the Lord's Supper are the biblical acts designed for that purpose. And so when a person believes the gospel and submits to baptism, the experience of baptism represents an exercise of faith that encompasses the visible and tangible realities of a bodliy experience of going underwater and coming back up to a new kind of life. When a person eats the bread and drinks from the cup in faith, the gospel has been applied to him or her by a personal, embodied action that involves multiple senses of sight, touch, and taste. Through these bodily acts, faith goes public, and in going public, the Holy Spirit strengthens that faith.
Suggested passage for personal or family reading: 1 Corinthians 10. In the context of warning the Corinthians not to participate in pagan feasts, what does Paul teach about the Lord's Supper? What were the "ordinances" that Israel had in the wilderness, and why didn't those ordinances ultimately save them from death? What lessons should we learn from that example about ordinances in the new covenant?