144,000 from all the tribes of Israel were sealed

Revelation 7:1-8

After this I saw four angels standing at the four corners of the earth, holding back the four winds of the earth to prevent any wind from blowing on the land or on the sea or on any tree. Then I saw another angel coming up from the east, having the seal of the living God. He called out in a loud voice to the four angels who had been given power to harm the land and the sea: Do not harm the land or the sea or the trees until we put a seal on the foreheads of the servants of our God.” Then I heard the number of those who were sealed: 144,000 from all the tribes of Israel.

The four winds may be the same as the four horsemen of chapter 6. If so, John sees the forces of destruction held back while God sets his mark of ownership on everyone who belongs to him. 

144000 A Symbolic number: the total of all God's people(12x12x1000), identical with the 'great multitude. See 'Numbers with special meaning. We take Israel to mean, not the nation, but God's people. Dan is omitted from this list. Usually, both Joseph and his son Manasseh are included (but not Josph's other son, Ephraim)

These people are specifically referenced only twice in the Bible. In the end time, after God commands a temporary ceasing of earth's calamities (Revelation 6, 7:1 - 3), he sends a powerful angel on a special mission. The angel is not to allow the sea or even earth's trees to be damage until a unique group of folks are set apart.

The 144,000 are mentioned again later in Revelation. The Apostle John, in a vision, sees a group of resurrected believers standing with Jesus Christ. They have been called and converted by God during the great tribulation period.

https://www.biblestudy.org/bibleref/meaning-of-numbers-in-bible/144000.html

Why?

Dan and Manasseh are not listed in the 144000. For their rebellions and worship idols,

How?

God's people will be protected from harm on the day of judgment.