Stone Detailing is the labor required to smooth and engrave stone floors and walls, and to create fortifications. Detailing will increase room value but is considered a time consuming process by some, compared to other tasks. The surface must be smoothed first, and can then be designated to be engraved or made into a fortification. It is not possible to engrave constructed walls and floors, even though engraved walls that could not possibly be natural can be seen in temples in adventure mode.
Engraving is a purely optional task, not necessary for survival. It is, however, an easy way to add value to a room and, for highly trained engravers, can depict events from the history of your fort. These engravings are more detailed when you visit them in adventurer mode, unless you edit the init file for dwarf fortress.
Stone detailing is a good way to train attributes for your dwarves before entering them into the military.