A flat is an image that is drawn on the floors and ceilings of sectors. Flats are very different from wall textures. Flats are a raw collection of pixel values with no offset or other dimension information; each flat is a named lump of 4096 bytes representing a 64x64 square. The pixel values are converted to actual colors in the same way as for the Doom picture format.
Placing flats in PWAD files presents a number of difficulties.
Complete details are in the Unofficial Doom Specs.