You've received a view replies already, but I figure it's worth clarifying.
That's the tempo, i.e. the beats per minute (bpm). In accordance with the note type, that's 120 quarter-notes per minute (2 per second), but since this is in a 6/8 time signature the bars (space between each vertical line) are measured in 6 eighth-notes (the weight of each note is different than in 3/4 time). So with the first lyrics, "We sing through dark of coldest night" takes 3 seconds even though it happens across 2 bars.
So, if you listen to this metronome
The distinct little "bong" signifies the start of each bar in the sheet music, and is usually a more emphasized lyric, such as the words "sing", the word "dawn", "gone", etc.
If you sing along like it sounds in their music video, you'll probably find that it matches perfectly.