For me, it’s the following sentence that gives the meaning to that one.

The countryside is literally so beautiful that the beauty cannot (be) beheld [sic]. That is, the sunlight hitting that bit of the world at that bob of time is so intense that the viewing is indistinguishable in its ordinary pieces because of the sheer pleasure. The view is just a singular whole all at once, because the thick and luminous real is everywhere and dripping off of every thing. The abundance drowns out the particulars.

You’re squinting, but not because of pain.

I understand it as sunlight so bright that your eyes are half-closed, obscuring the landscape