This is the first two lines of the 27th paurhi in Jap ji:
so dar kehaa so ghar kehaa, jit beh sarab samaale
It's such a profound paurhi it's repeated, with slight variations, in Siri Guru Granth Sahib after Jap Ji and before other shabds.
What Guru Nanak is doing is asking a very profound question: He's saying to the Divine:
'Where is that door, where is that home, where You are sitting and actually able to look after everything and support everything'.
In other words: We know there's a macro-consciousness, and we know there is a micro reality. This is something you find in many different philosophies. Maybe God created the universe but then just let's it go on it's own way. Or: I just believe in the accident of evolution, I can't concieve of a Divine power that's looking at everything going on.
Guru Nanak is saying 'How do You do it!? How is it that You are both Formless/Timeless/All-Knowing/All-Seeing... AND actually connected to and guiding the Creation? Where is that place where you're looking after everything on a practical level?'
And the answer comes in the next line:
vaaje naad anek asankha, kete tere vaavanhaare
Guru Nanak gives a detailed practical answer to this philosophical problem. The easiest way to say is that GOD HAS A BAND! Creation has a band.
'All of these musical instruments that are vibrating, there's so many of them they are countless, and there's so many musicians that are playing them.'
It gives a vision that the interface between Creator and Creation is sound, is music. When we're wanting to be in a communication with the Creator or the Creator wants to be in communication with us, there are so many countless, and there are so many musicians. This is the field of the creative play between the mundane and the unfathomable. Between the All that is both Nothing and All-Knowing and temporary, beautiful, micro plays that happen... there is a song. The song is the thing that's guiding Creation and the song is the thing that's guiding us back to the Creator.
My strong feeling about this is that the reason the 'so dar' paurhi gets repeated after Jap Ji in the Siri Guru Granth Sahib is because of this idea that if we want to progress as humans, we have to listen to, recieve and play with the naad/sound. That we need to become musical instruments ourselves, through which we can reach the Creator and through which the Creator can move us to be a conscious and loving part of the Creation Itself.
That's why the entire Siri Guru Granth Sahib itself, it may be poetry (and it is!), it may be philosophy (and it is!), it may be interfaith (and it is!), but mostly it's music. Mostly it's song. Mostly it's the song to guide us home, and it's the song to help us bring the heaven's to the earth.