SAVIOUR= A saviour is someone who saves people’s life (in a physical or symbolical way). Christians believe that Jesus is our Saviour because with his life and death, he can save people from evil and so, give us eternal life after death.

HAVE= This auxiliary verb appears several times in the song and it's always pronounced /əv/, without H. This is normal, though it's much more normal to pronounce just /v/ (as in "we've been").

BE THROUGH= If you have been through something, that period is finished.

HARVEST= If you grow something in the fields, for instance corn, the harvest is the time when you collect the cereal from the fields, or the cereal you collect.

THE CHILL= The cold. The chill of the night is the low temperatures of the night. When it’s cold, you can also say that it’s chilly.

MANY HAVE COME... = Many people have come.

ON SAVIOUR’S DAY= Remember that we use ON with days and dates. So, if we say "on Christmas" we are referring to the day, and if we say "in Christmas" we are referring to the season. "Saviour’s Day" is the way he calls "Christmas Day". The word "Christmas" comes from Old English "Crīstes mæsse", which means, the mass of Christ ("mass" is a catholic religious service) so in the beginning it was used for Christmas Day, but today we can also use it for the holiday time between Christmas Eve (the day before Christmas) till New Year’s Day. For all the holiday we can also say "Christmas season" or "Christmastide" (in Old English "tide" means "time"), and we can also say "Yuletide" (a pre-Christian name) or simply "Yule", which can be both the season or the day.

HE IS CALLING YOU= The pronoun HE refers to Jesus (the Saviour).

THE OLD= In this song we have a few adjectives used with an article, so they are not really adjectives but nouns: the old, the black, the white. We can do this with some adjectives when we refer to a group of people with that quality:
The old = old people.
The black = black people.
The rich and the poor = rich people and poor people.
We can also say "the young" (= young people) in this sense, but in the song he says "the young ones", and here, the noun is ONES (a pronoun) and YOUNG is just a simple adjective that goes before ONES.

MEETING THE NEED OF THE HUNGRY IS HE= If you meet a need, you satisfy it, so this sentence means: He (Jesus) is satisfying the need of everybody who is hungry (the hungry = hungry people). Here, "hungry" doesn’t refer to physical food, but spiritual food. The order of the sentence has been altered, it should be "he is meeting...", but in poetry (and music) we can do this.

HERE’S TO= We use this expression when we want to have a toast for somebody or something (a toast is when we drink to honour somebody). So, when we are drinking in a celebration and we want to honour John or drink to the health of John, we raise our glass and say "here’s to John", and then we drink (or first we lightly touch glasses and then we drink).