Who Are The 144,000?

4 I heard the number of those who were sealed, one hundred forty-four thousand, sealed out of every tribe of the sons of Israel:
5 of the tribe of Judah were sealed twelve thousand,
of the tribe of Reuben twelve thousand,
of the tribe of Gad twelve thousand,
6 of the tribe of Asher twelve thousand,
of the tribe of Naphtali twelve thousand,
of the tribe of Manasseh twelve thousand,
7 of the tribe of Simeon twelve thousand,
of the tribe of Levi twelve thousand,
of the tribe of Issachar twelve thousand,
8 of the tribe of Zebulun twelve thousand,
of the tribe of Joseph twelve thousand,
of the tribe of Benjamin were sealed twelve thousand.

 Revelation 7:4-8

What Do The Numbers Mean?