A clerestory is a series of windows high up in a space. These can be a formal row, like in a cathedral, or a single, simple opening in the wall. Clerestories are often confused with transoms. Transoms are smaller windows stacked on top of other windows or doors. In traditional buildings, transoms were operable allowing air to move around a building while still keeping the safety and privacy of closed lower doors and windows.
Clerestories really have two functions. The first is obviously to bring in light from above. This kind of high light allows for illumination without sacrificing privacy, similar to the description of transom given above. Clerestories also significantly lighten the weight of tall walls be substituting glass for heavier materials. In the stone cathedrals of Western Europe, that combined function - high illumination and reduced weight - was an essential ingredient in the drive for increased height reaching up to a sunlit heaven.