The TEXTURE1 and TEXTURE2 lumps define how wall patches from the WAD file should combine to form wall textures. In other words, a texture is decomposed into smaller textures in such a way that repeating areas are only stored once. At startup time (to be precise, during "R_Init: Init DOOM refresh daemon"), the patches inside the WAD are read and combined into Wall Textures, or how the source code refers to, 'map textures'.