It means Japan Domestic Market. It was a term coined for car parts from Japan.
In the fish world JDM is a style of fish keeping based on the way a lot of people in Japan keep fish. It basically means to keep a lot of large fish in a small tank, usually way over stocked.
Basically it means that they set them up like the tanks in the Japenese Domestic Market. So they typically keep them with no substrate or decorations and large amounts of fish.