Reformed theology recognizes two sacraments instituted by Christ in the New Covenant - Baptism and the Lord's Supper. We refer to them as sacraments instead of mere "ordinances" because the Bible attributes more to them than "naked signs" (cf. Gal 3:27, 1 Cor 10:16). They are not just acts of obedience that stir-up our collective memories, they are mysterious (sacramentum) gifts from God to His people.
Reformed folk often define sacraments as "signs and seals of the covenant of grace" based on the language of Romans 4:11,
"And he received the sign of circumcision, a seal of the righteousness of the faith which he had..."
Understanding sacraments as signs is pretty easy - they mark-out the recipient as a member of God's covenant people. They signify that the person is no longer his own - the sacrament functions as a badge of covenant membership.
Unfortunately, many Reformed folk define a "seal" in the same way they define a sign - it is an official "stamp" or insignia impressed on a person - validating their covenant membership. You may have heard of how royal "signet" rings were pressed into wax to "seal" letters in order to authenticate them... So far so good, but we shouldn't stop there. Seals have a signatory aspect, but they have more - they close or protect documents so that they are opened properly. There is no better place to show this than Revelation where the scroll with seven "seals" could only be opened by the Lamb (Rev 5). The scroll was protected - it was "sealed" so that it would not be opened improperly.
The cash value of this teaching is that sacraments serve as mysterious means of "sealing" / keeping us for God in His economy. Baptism is not just a sign that we are "externally" part of the visible church (WCF 28:1), it is a spiritual way of sealing us to God. Baptismal water does not "magically" regenerate in and of itself (ex opere operato), but the Spirit "mysteriously" works to bind us to Himself in and through a faith bathed in the sacrament(s), not devoid of them.
In the Gospel we are delivered, but we are also signed and sealed. The Gospel is not an ethereal reality that stands outside of us - it is an incarnate Person that we are united to as evidenced by God's sacramental economy.