Our booklet The Book of Revelation Unveiled explains:
Who are these 144,000? How are they identified?
"And I heard the number of those who were sealed. One hundred and forty-four thousand of all the tribes of the children of Israel were sealed" (Revelation 7:4 Revelation 7:4And 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.
American King James Version×). This sealing has to do with receiving God's Holy Spirit so one may become spiritually converted. As Ephesians 1:13-14 Ephesians 1:13-14  In whom you also trusted, after that you heard the word of truth, the gospel of your salvation: in whom also after that you believed, you were sealed with that holy Spirit of promise,  Which is the earnest of our inheritance until the redemption of the purchased possession, to the praise of his glory.
American King James Version×says: "Having believed, you were marked in him with a seal, the promised Holy Spirit, [which] is a deposit guaranteeing our inheritance until the redemption of those who are God's possession" (NIV, compare Ephesians 4:30 Ephesians 4:30And grieve not the holy Spirit of God, whereby you are sealed to the day of redemption.
American King James Version×; 2 Timothy 2:19 2 Timothy 2:19Nevertheless the foundation of God stands sure, having this seal, The Lord knows them that are his. And, Let every one that names the name of Christ depart from iniquity.
American King James Version×).
The 144,000 of Revelation 7 are mentioned again in Revelation 14, where it becomes clear that these spiritual "firstfruits" will have repented and been converted before the beginning of the Day of the Lord. They are described as being redeemed and without fault. They are represented as having a relationship both with God the Father and Jesus Christ as the Lamb of God.
"Then I looked, and behold, a Lamb standing on Mount Zion, and with Him one hundred and forty-four thousand, having His Father's name written on their foreheads. And I heard a voice from heaven, like the voice of many waters, and like the voice of loud thunder. And I heard the sound of harpists playing their harps. They sang as it were a new song before the throne, before the four living creatures, and the elders; and no one could learn that song except the hundred and forty-four thousand who were redeemed from the earth.
"These are the ones who were not defiled with women, for they are virgins. These are the ones who follow the Lamb wherever He goes. These were redeemed from among men, being firstfruits to God and to the Lamb. And in their mouth was found no deceit, for they are without fault before the throne of God" (Revelation 14:1-5 Revelation 14:1-5  And I looked, and, see, 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.
 And I heard a voice from heaven, as the voice of many waters, and as the voice of a great thunder: and I heard the voice of harpers harping with their harps:
 And they sung as it were a new song before the throne, and before the four beasts, and the elders: and no man could learn that song but the hundred and forty and four thousand, which were redeemed from the earth.
 These are they which were not defiled with women; for they are virgins. These are they which follow the Lamb wherever he goes. These were redeemed from among men, being the first fruits to God and to the Lamb.
 And in their mouth was found no guile: for they are without fault before the throne of God.
American King James Version×).