7:1 After this, I saw four angels standing at the four corners of the earth,
holding the four winds of the earth, so that no wind would blow on the earth,
or on the sea, or on any tree.
7:2 I saw another angel ascend from the sunrise, having the seal of the living God.
He cried with a loud voice to the four angels to whom it was given to harm the earth and the sea,
7:3 saying, "Don't harm the earth, neither the sea, nor the trees, until we have sealed the bondservants of our God on their foreheads!"
7:4 I heard the number of those who were sealed,
one hundred forty-four thousand, sealed out of every tribe of the children of Israel:

7:5 Of the tribe of Judah were sealed twelve thousand,
Of the tribe of Reuben twelve thousand,
Of the tribe of Gad twelve thousand,
7:6 Of the tribe of Asher twelve thousand,
Of the tribe of Naphtali twelve thousand,
Of the tribe of Manasseh twelve thousand,
7:7 Of the tribe of Simeon twelve thousand,
Of the tribe of Levi twelve thousand,
Of the tribe of Issachar twelve thousand,
7:8 Of the tribe of Zebulun twelve thousand,
Of the tribe of Joseph twelve thousand,
Of the tribe of Benjamin were sealed twelve thousand.

7:9 After these things I looked, and behold, a great multitude, which no man could number,
out of every nation and of all tribes, peoples, and languages,
standing before the throne and before the Lamb,
dressed in white robes, with palm branches in their hands.
7:10 They cried with a loud voice, saying,
"Salvation be to our God, who sits on the throne, and to the Lamb!"

7:11 All the angels were standing around the throne, the elders,
and the four living creatures; and they fell on their faces before his throne, and worshiped God,
7:12 saying, "Amen! Blessing, glory, wisdom, thanksgiving,
honor, power, and might, be to our God forever and ever! Amen."

7:13 One of the elders answered, saying to me,
"These who are arrayed in white robes,
who are they, and from where did they come?"
7:14 I told him, "My lord, you know." He said to me,
"These are those who came out of the great tribulation.
They washed their robes, and made them white in the Lamb's blood.
7:15 Therefore they are before the throne of God,
they serve him day and night in his temple.
He who sits on the throne will spread his tent over them.
7:16 They will never be hungry, neither thirsty any more;
neither will the sun beat on them, nor any heat;
7:17 for the Lamb who is in the midst of the throne shepherds them,
and leads them to springs of waters of life.
And God will wipe away every tear from their eyes."

The twelve tribes, each with 12,000, may symbolize a complete count of the believers. The tribes may represent the diversity in the body of Christ, whether by giftedness or denomination. The New Testament treats the Christians as the true Jews in a spiritual sense, so I don't think that this is referring to Hebrew-Christians in particular, for Revelations is a vision and to what extent it is to be taken literally is uncertain. The tribes are not mentioned in their normal order. For the tribe of DAN is left out. Joseph normally contains Ephraim and Mannaseh, which were his sons. But apparently only refers to Ephraim and Mannaseh is mentioned separately. Levi is also mentioned, which if Dan were included would make 13.

Why was Dan left out? Perhaps one could draw a parallel with the twelve apostles. One was apostate - Judas - and replace by Paul, in my opinion, although the other apostles tried to replace him with a man of their own choosing. Perhaps Dan represents the apostate Christians, who will not be reckoned among the redeemed.

An early Church Father - Irenaeus (150 ad) - holds that Dan is left out because he believes the Anti-Christ is from that tribe.:

"Jeremiah does not merely point out his sudden coming (of the anti-Christ), but he even indicates the tribe from which he shall come, where he says, “We shall hear the voice of his swift horses from Dan; the whole earth shall be moved by the voice of the neighing of his galloping horses: he shall also come and devour the earth, and the fulness thereof, the city also, and they that dwell therein.” This, too, is the reason that this tribe is not reckoned in the Apocalypse along with those which are saved."

As with the elders, their worship is also directed both to God and to the Lamb of God.

"Washed their robes" is an active verb implying some action on their part. It is used figuratively of those who by faith so appropriate the results of Christ's expiation as to be regarded by God as pure and sinless.

"Serving Him day and night" is contrary to some views about heaven being simply a place where people do whatever they felt like doing on earth. It is a place of service. On the other hand the impression that heaven is a pleasant place, free of pain and suffering and sorrow, is correct. The Islamic concept of heaven is that the men would be lying around on couches, being ministered to by virgins. (I guess the women wouldn't get much out of it). That seems rather vain. The Christians continue to be actively involved in serving God even in heaven.