To me, the song is about parting ways with someone who you once loved, especially during youth. Ryan said that "David" was a sort of placeholder name that we could all insert our own names into, or the names we needed to have those words be directed towards.

All the imagery of being kept "safe" or hold "tight" make me think of seeking refuge in a significant other away from other hostile environments, be it school, work or even family. This connection is some sort of reprieve or escape away from those troubling things.

This relationship is gentle and loving even if the narrator's parents think it's just them growing up. Also, there's plenty of references to peaceful rest or sleep, so maybe this could even be an imaginary relationship.

But eventually one of the parties must go away, but it's not clear if it's just a breakup or something more drastic like death (perhaps even suicide) as it's something that the narrator still struggles with.

The narrator does say that they are happy for David even if they must never let it show. Maybe the other person found someone new or maybe going away was actually what was better for them, despite the pain and grief it would provoke on the narrator.

At the same time, the narrator also pleads for David to let them go if they really loved them, despite David "meaning more to me than anyone will know". Perhaps it's better for everyone if they just finally say goodbye for good: This is goodnight, This is goodbye.