In days of old, a king would seal a special letter with his royal seal stamped in wax. This let everyone know that the letter was truly from the king, contained his word, and was to be guarded and kept safe.
In the same way, (though spiritually, and not with wax!) each new believer is "sealed" with the Holy Spirit, to show the truth and genuineness of his new life, and as a promise from God of what is to come in the future, when Jesus comes back again.

And grieve not the Holy Spirit of God, whereby ye are sealed unto the day of redemption.
Ephesians 4:30