What is spelt flour as used in the Blueberry Galette?

Spelt is a species of wheat, Triticum spelta, that is believed to have been cultivated for thousands of years, and is sometimes considered a subspecies of ordinary wheat rather than a separate species. It is not the same as White Whole Wheat Flour, but can be used interchangeably with it.
https://www.thespruceeats.com/what-is-spelt-flour-5202073

Spelt flour is also called White Whole Wheat Flour. It is a whole grain flour that is healthier than all purpose flour. I find it has a nuttier flavour. When swapped for AP flour in a recipe, remember that it takes a few minutes longer to absorb liquids.