This hymn is a Christmas carol written by Scottish poet James Montgomery in 1816. This poem is usually sang in England to the tune of the carol "Angels We Have Heard on High" (which in turn borrows the tune from the French carol Les anges dans nos campagnes"). In the United States it is usually sang to the tune of Regent Square, but on this video they use the British tune instead. Sometimes, like in this case, they use the chorus from "Angels We Have Heard on High" instead of the original refrain from the poem, which goes like this:

Come and worship,
Come and worship
Worship Christ, the newborn King.

Angels, from the realms of glory,
Wing your flight o'er all the earth;
Ye who sang creation's story,
Now proclaim Messiah's birth:

Gloria
Gloria

Gloria in Excelsis Deo
Gloria in Excelsis Deo

Gloria
Gloria
Gloria
Gloria

Saints before the altar bending,
Watching long in hope and fear,
Suddenly the Lord, descending,
In his temple shall appear.

Though an infant now we view him,
He shall fill his Father's throne,
Gather all the nations to him;
Every knee shall then bow down.

Gloria in Excelsis Deo
Gloria in Excelsis Deo

Gloria
Gloria and love

REALM= /relm/ Kingdom (especially in a figurative sense)

WING= The part of a bird they use to fly. In this case it is used as a verb, so it means "fly (using your wings)".

O'ER= (old fashioned or poetic) /əʊ/ Over.

Ye= (old fashioned or poetic) /ji:/ You. In old English "ye" is the subject and "you" is the object (pair ye-you like I-me or he-him), later the form "you" got used for the subjet too.

PROCLAIM= (literary) Announce.

MESSIAH= According to the Jews, the Messiah was the man God promised to send to the world to save us. Christians beleive that Messiah is Jesus.

GLORIA IN EXCELSIS DEO (in Latin)= Glory to God in the highest (= in heaven)

BENDING= Bending over, making a reverence as a sign of respect or worship.

FEAR= Religious fear is not a bad thing (as the fear of a wolf), but rather a kind of strong emotion mixed with awe, amazement, when you experience the power of God or the supernatural.

SHALL= (old fashioned or literary) Will.

INFANT= child.

VIEW= See.

GATHER= Bring together.

BOW DOWN= To make a reverence as a sign of respect or worship. To bow down your knee is to come down to your knees.