The word Advent derives from the Latin adventus, which means an arrival or visit. Advent is the beginning of the spiritual year for these churches, and it's observed the four Sundays before Christmas Day.
Advent, advent,
a light burns.
First one, then two
then three, then four,
Then the Christ Child is at the door.
The poem is about the Advent season. The content alludes to the tradition of lighting one more candle on the Advent wreath on each of the four Sundays in Advent in order to announce the approach of Christmas through the increasing brightness.