I had no idea.
So I searched SCRIPTURE.
And here's what Scripture had to say:
Ephesians 1:13 (King James Version)
13 In whom
ye also trusted,
after that
ye heard
the word
of truth,
the gospel
of your salvation:
in whom also
after that
ye believed,
ye were sealed
with that holy Spirit
of promise,
Ephesians 4:30 (KJV)
30 And grieve not
the holy Spirit
of God,
whereby ye
are sealed
unto the day
of redemption.
2 Corinthians 1:21-22 (KJV)
21 Now he
which stablisheth us
with you
in Christ,
and hath anointed us,
is God;
22 Who hath
also sealed us,
and given
the earnest
of the Spirit
in our hearts.
2 Corinthians 5:5 (KJV)
5 Now he
that hath wrought us
for the selfsame thing
is God,
who also
hath given
unto us
the earnest
of the Spirit.
John 6:27 (KJV)
27 Labour not
for the meat
which perisheth,
but for that meat
which endureth
unto everlasting life,
which the Son
of man
shall give
unto you:
for him
hath God
the Father
sealed.
Revelation 7:3-4 (KJV)
3 Saying,
Hurt not the earth,
neither the sea,
nor the trees,
till we
have sealed
the servants
of our God
in their foreheads.
4 And I heard
the number
of them
which were sealed:
and there were sealed
an hundred
and forty
and four thousand
of all the tribes
of the children
of Israel.
Revelation 9:4 (KJV)
4 And it
was commanded them
that they
should not hurt
the grass
of the earth,
neither any green thing,
neither any tree;
but only those men
which have not
the seal
of God
in their foreheads.
Revelation 14:1 (KJV)
14 And I looked,
and, lo,
a Lamb stood
on the mount Sion,
and with him
an hundred forty
and four thousand,
having his Father's name
written
in their foreheads.
Revelation 22:4 (KJV)
4 And they
shall see
his face;
and his name
shall be
in their foreheads.
Isaiah 42:1 (KJV)
42 Behold my servant,
whom I uphold;
mine elect,
in whom
my soul delighteth;
I have put
my spirit
upon him:
he shall bring forth
judgment to the Gentiles.
Additional material:
(Source: Knowing Jesus)