In the dictionary the word 'seal' is described as: to close or fasten with a seal.
When you heard the good news of your salvation and believed it, you were sealed with the Holy Spirit of promise.
The word "sealed" in Bible terminology means: to stamp with a signet or private mark. Or a die or signet having a raised or incised emblem used to stamp an impression on a receptive substance such as wax or lead thus an impression so made.
It means to fasten us up by the Holy Spirit and keep us as a top-secret prize, safe from the enemy. It is a sign of authority, given by God Himself.
Paul says in Ephesians 1:13 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.
The Message Bible says: It's in Christ that you, once you heard the truth and believed it (this Message of your salvation), found yourselves home free—signed, sealed and delivered by the Holy Spirit.
It's because you heard and believed the message of truth and put your faith in Christ that you were given the promised Holy Spirit to show everyone that you belong to God.
God has put His stamp of ownership and approval on you by giving you the Holy Spirit and now you can live for Him according to the promise.
Today can you believe that as a child of God He has sealed you with the Holy Spirit because you believe in Jesus!