A seal in Scripture can refer to a royal signet ring, used to enact a decree (cf. Esther 8:10).
In the New Testament, God seals his believers to mark his ownership of us (2 Cor 1:22) by the Holy Spirit (Eph. 1:13-14).
2 Corinthians 1:22 ~ Now it is God who makes both us and you stand firm in Christ. He anointed us, set his seal of ownership on us, and put his Spirit in our hearts as a deposit, guaranteeing what is to come.
Ephesians 1:13-14 ~ And you also were included in Christ when you heard the word of truth, the gospel of your salvation. Having believed, you were marked in him with a seal, the promised Holy Spirit, who is a deposit guaranteeing our inheritance until the redemption of those who are God's possession—to the praise of his glory.
Adherants of reformed theology believe that circumcision and baptism are both signs and seals of the covenant of grace.
Romans 4:11 ~ And he received the sign of circumcision, a seal of the righteousness that he had by faith while he was still uncircumcised. So then, he is the father of all who believe but have not been circumcised, in order that righteousness might be credited to them.