Metopes are the areas between the triglyphs. It's thought that in the earliest (proto-doric) temples, that there were empty spaces between the beams resting on the architrave. This was a problem, as these empty spaces attracted birds, who would nest there and make messes on the steps below. The solution in the earliest doric temples was to fill these spaces with mud-brick. And then, of course, to decorate these spaces. We know that some early temples had painted ceramic plaques (metopes) between the triglyphs, and later, when doric temples were made of stone, the metopes were panels of relief sculpture.