Basically during the tribulation, 144,000 male virgins will go out and preach gospel, these men are made up of 12,000 from each of the twelve tribes. see Rev 7 below.
Basically where are these tribes today? Many tribes are considered missing and have been missing for thousands of years. SO we have all the usual nonsense speculation and hypothosis about the missing tribes, even saying that tribes went off to America and some are found in UK etc.
Truth is we have no idea, however there are now DNA testing programmings running on Jews to build up a database to try and identify people from different tribes, someone I know had his DNA tested and its come back that he is 100% Levite, which he was encouraged by because he has always had a yearning for priestly duties etc.
God of course knows who these people are, so we just let God get on and pick the 12,000 from each tribe even if we still have no idea as to the identities of these tribes.
Revelation 7
7 And after these things I saw four angels standing on the four corners of the earth, holding the four winds of the earth, that the wind should not blow on the earth, nor on the sea, nor on any tree.
2 And I saw another angel ascending from the east, having the seal of the living God: and he cried with a loud voice to the four angels, to whom it was given to hurt the earth and the sea,
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.
5 Of the tribe of Juda were sealed twelve thousand. Of the tribe of Reuben were sealed twelve thousand. Of the tribe of Gad were sealed twelve thousand.
6 Of the tribe of Aser were sealed twelve thousand. Of the tribe of Nephthalim were sealed twelve thousand. Of the tribe of Manasses were sealed twelve thousand.
7 Of the tribe of Simeon were sealed twelve thousand. Of the tribe of Levi were sealed twelve thousand. Of the tribe of Issachar were sealed twelve thousand.
8 Of the tribe of Zabulon were sealed twelve thousand. Of the tribe of Joseph were sealed twelve thousand. Of the tribe of Benjamin were sealed twelve thousand.
9 After this I beheld, and, lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands;
10 And cried with a loud voice, saying, Salvation to our God which sitteth upon the throne, and unto the Lamb.
Revelation 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.
2 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:
3 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.
4 These are they which were not defiled with women; for they are virgins. These are they which follow the Lamb whithersoever he goeth. These were redeemed from among men, being the firstfruits unto God and to the Lamb