A well-known (= famous) song, that was first recorded in 1962 by a singer named Ben E. King. This song has been recorded many many times by different artists. This version of the song is by a contemporary (= happening now) band, 'Imagine Dragons'.
The song is quite repetitive, but it is a gentle song which has a beautifully simple tune, and the lyrics are sweet.
To 'stand by' someone is a metaphor. We use a lot of metaphors in English - this is when words or phrases do not have their real or literal meaning, but have a different meaning.
When we say someone stands by us, we mean they support us.
We mean that they stay with us when we are in times of trouble or difficulty.
So, in this song, the songwriter talks about the dark and the night. These are words that in English can be used as metaphors to describe danger or difficulty. He says 'I won't be afraid as long as you stand by me. (As long as = 'if')
So, this means 'I won't be afraid, if you support me'
Then he talks about more disaster and danger (the sky tumbles and falls / the mountain crumbles (= to break apart) and that if that happens he won't cry or shed a tear (collocation - when we cry, we shed tears) if his friend stands by him (supports him).
He then offers his support to the other person by saying 'whenever you are in trouble' then 'stand by me' (I will support you).
Vocabulary: well known, contemporary, stand by, metaphor, literal, as long as, crumble, shed