The song is heard during the process where Branch accepts that he will never complete the puzzle him and his friends are working on, and the Trolls set about restoring order back into his bunker. Thus as the topic of the song suggests they help him "pick up the pieces" of his life.
Lyrics
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Things fall apart
We lose our way sometimes
But you are never on your own
Know in your heart
I'm always by your side
I know it feels like I am gone
But when you feel alone
Just look right over your shoulder
And find the ones that love you
Holding out their hands
To help you pick up the pieces
It isn't easy but you got your friends
They'll help you pick up the pieces
They won't be leavin'
'Til you're found again
And, honey, oh, I know
It's heavy to hold
But you pick up the pieces
it isn't easy, but it always ends
And you got your friends
Trivia
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In her bio for Trolls, Grandma Rosiepuff had her voice described as "Her soothing songs can lull even the fussiest troll into a calming nap". The song fits within the description of her voice, as it is calm and lullaby-like at points. Thus, this song is probably sung by a flashback version of her.