It's a poetic expression that could have several interpretations depending on the context:
* A source of divine inspiration or revelation: The mouth of heaven could represent a channel through which God speaks or reveals his will.
* A symbol of hope and salvation: A mouth opening up in the sky could be seen as a sign of hope and salvation, a promise of a better future.
* A metaphor for a powerful voice: The phrase could also be used metaphorically to describe a voice that is incredibly powerful or influential, as if it were coming from heaven itself.
Without more context, it's hard to pinpoint the exact meaning. It could be part of a poem, a song lyric, a painting title, or even a philosophical concept.
If you can provide more information about where you encountered this phrase, I might be able to give you a more precise interpretation.