In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise. Ephesians 1:13
A seal is like a stamp, a signature, or a logo. When a seal is placed on something it signifies ownership, guarantee, and security. In Christ, Jesus God has put His Holy Spirit in us as His seal of ownership, guarantee, and security. This means we belong to God, all his promises to us in Christ are guaranteed and our salvation is also secured in Him.