The Missing textures is a black and magenta checkered pattern that happens when you don't have the textures for a map, weapon, prop or NPC or that you don't have a Source game mounted (e.g Counter-Strike: Source) into Garry's Mod.
Description[]
This texture looks like this to distinguish normal textures from the missing texture. Normally textures don't look like this so it's easier to catch one.
Gallery[]
An example of missing textures when you don't have the textures for the map.