If hair or fur is matted, it is stuck together in thick clumps, like a rough mat on the floor. You cannot run a comb through it easily. You see it on a dog that has not been brushed, on a child after the sea, on wool that has felted. It is almost always about hair, fur, or fibres. Grass can be matted after rain; leaves can lie in a matted layer. The picture is the same: many thin things pressed and twisted into one mass. A quieter second use: a matted photograph is one set in a cardboard border (a mat) before it goes in a frame. That is a different picture from tangled hair, but the same spelling. Do not mix any of this with matte (often one t in American spelling, or matte as in paint). Matte paint is not shiny. Matted hair is tangled. They sound almost the same.