I've blogged about the scene before and the blog post for it seems pretty popular on here. Long story short, the kid Kevin is left behind by his family on Christmas and goes to the local church and sits in a pew as he listens to the choir sing. He is joined there by Old Man Marley, an old man of whom he is afraid. However, there the two bond and from that point on Kevin is no longer afraid of him.
Interestingly enough, Home Alone and A Bad Moms Christmas are very similar. Both are comedy films with slapstick (and in Bad Moms's case, raunchiness as well) and yet have this quiet moment to form solace between two characters.