It means the bus shelter is made out of red sandstone, which can be tagged on it using building:material=sandstone building:colour=red (see for example https://wiki.openstreetmap.org/wiki/Key:building:material). It’s not critical that the material/colour are tagged, but I included them in my note because it’s quite unusual to see a stone-built bus shelter, and because red sandstone is quite particular to northern Cumbria.

I don’t think your recent edits (103488526, 103488872) to delete the shelter building and stop_area relation are correct. There’s definitely a bus shelter building there, and I believe it should be mapped as it occupies space.

What was your thinking behind deleting the shelter building? Ta