This picture shows a pretty severe case of filtration lines. Filtration lines will form when air is forced to flow through fibers including carpet fibers and consequently they act as a filter filtering chemical impurities and particles out of the air. Usually they’ll form under doorways and along the edge of carpet near walls. In this instance there was a heat register under a nightstand that forced air under the nightstand and is it try to escape it was flowing through the carpet fibers around the nightstand. Filtration lines are not easily removed with normal carpet cleaning. They require extra time attention and special chemicals and still may not be entirely removed.