Derelict Freighters are abandoned semi-destroyed freighters that are full of loot for you to take. You need to talk to the scrap dealer NPC on any space station to buy (5.000.000 units) coordinate data that you will use while in your ship. Then enter pulse drive and exit when prompted. You will be at a derelict freighter. Land on it and go explore.
Be careful because some can be dangerous. You can get all kinds of valuable stuff, and you will also notice that the price for coordinate data increases if you want to buy more. To reset this, simply go to another system and talk to another NPC (his price will be lower).