Unfinished concrete, specifically. I would translate "brut" as "bare" or "unrefined" here. English brutality is related to French brut, but in French it means more like crude, unrefined, raw, blunt, with no particular sense of animalistic or violent.
Brut is raw. Brute or brutal is related to violence.
Perhaps we should be right-click->view-source'ing for the true brutalist experience.