Impassable tiles are tiles in a building that creatures cannot move into--they function similarly to a constructed wall. Some workshops, like the bowyer's workshop and jeweler's workshop, have what is in effect a three square wall down one side. Non-workshop buildings (such as furnaces and screw pumps) may also include impassable tiles.
Impassable tiles appear as a dark green color during initial placement (passable tiles are shown in light green). One common use for impassable tiles is to block access to other z-levels. A magma smelter, for instance, has a single impassable tile in the top center--placing this tile over your magma access channel prevents unlucky dwarves from falling in, and unwelcome beasts from crawling out. It is worth noting that building destroyers can only destroy buildings that they can path to the z-level of. For machinery, the impassable tiles can be used to connect power through z-levels (see pump stack for an example).
Be careful when building workshops in tight quarters; more than one dwarf has perished from thirst after constructing a building that blocks the only path back to the fortress proper.