And I looked, and BEHOLD,
This is the same expression John uses in Revelation 4:1 in his first encounter with heaven. This implies that what he will describe is taking place in heaven.
THE LAMB was standing on MOUNT ZION,
The LAMB is such a contrast to the sea beast (Revelation 13:1) and the earth beast (Revelation 13:11). We sense a total change in the atmosphere is taking place in this verse.
This is the only place in the Revelation where ZION is used. The only other place where it is used in the New Testament is by Paul in Hebrews 12:18-29. Here, too, it has the heavenly qualities. The more I study the Revelation, the more I am convinced there is a heavenly reality (not fully experienced yet) and an earthly reality which actually is a subset of the heavenly reality. Thus, references in the Old Testament while referring to the city of Jerusalem also imply a foretaste of the heavenly ZION (e.g. Isaiah 59:20; Psalms 132:13-18; Psalms 2:4-6; Isaiah 2:2-4; Psalms 76:1-3).
and with HIM one hundred and forty-four thousand,
The tendency is to equate these 144,000 with those in Revelation 7:1-4. If these are the same 144,000 then this verse shows that the SEAL (Revelation 7:2) really works (Revelation 7:4), and these Jews are kept through all the terrible times to sing a great song (next verse). Dake's Annotated Reference Bible indicates that these 144,000 really are the man-child which was taken up into heaven (Revelation 12:5).
However, these 144,000 may also be a different group which meet the criteria in verses 4 and 5. We naturally assume these must be adults but they could be children like Jesus explained in Matthew 18:1-14.
having HIS NAME and the NAME of HIS FATHER written on their foreheads.
This verse confirms the overcomers at Philadelphia (Revelation 3:12). They also are the new Jerusalem occupants in Revelation 22:4.
The King James version omits HIS NAME (meaning Jesus' name) and only indicates the FATHER's NAME is written on their foreheads. The FATHER is 'dominant' in the triune GOD inasmuch Jesus calls the FATHER, "MY GOD", in John 20:17.
One thing is clear: this is a special group with a special praise (verse 3) and their purity is overwhelming (verse 4).