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

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

Ephesians 4:30