Homecoming is about a man who meets the love of his life and then she tells him this story about her coat getting dirty and her parents getting really angry at her. She ran away in the night. At the end she gives him a hug :) It is mainly about relationships and trust.
In the first stanza, it describes the game of trust where you fall back into your friends' arms - 'free-fall, backwards'.
In the second stanza, it describes how her coat was ruined by being 'blackened underfoot'. The way it says 'blackened', could also show her parents getting angry.
When it says 'Bed' it could be her parents telling her to go to her room.
'When you slip the latch and sneak' is alliteration with 's' sounds (sibilance) and sounds sneaky.
'These arms are sleeves' shows Simon giving her a hug. It refers back to the coat, and shows that they still love each other, and everything is now back to normal. As the coat 'still fits' it's like a reunion!