The phrase "dark grout" refers to the cement-like material used to fill the gaps between tiles. It's called "dark" because it has a color darker than the tiles, usually black or gray. Full definition
After renovating our bathroom this year and using a white mosaic tile with dark grout for the flooring, I wanted to carry that look into the kitchen as well. (lovegrowswild.com)
I knew right from the beginning that I wanted a white subway tile with dark grout for our backsplash. (lovegrowswild.com)