Some liquids, like blood or bile, are able to splash onto floors, furniture, items, and vehicle parts, and color them differently based on said liquid. The mop item (found or crafted via rags, some string and a plank) is able to clean these spillages up and revert the tile back to its original color. As far as I know it is the only item with this ability, unique in that it also serves as a great quality of life feature (reinforced by the fact it’s crafted from very common items and requires no skill or knowledge to do so).
a]ctivated, a prompt to select a dirty tile appears, then a direction is input and the tile gets cleaned up. The cleaning is instantaneous and does not use up turns, the in-game clock does not advance after the cleaning is done either. This seems to me like an oversight, as most in-game actions that require locomotion do use up in-game time, even if just a little bit of it.
On a different (not bug related) note, mopping dirty tiles should probably require water or soap, just like washing filthy items does. Alternatively, it could turn the mop itself filthy (if such a thing is even possible), requiring you to wash it before being able to use it again.