A JDN or Jump Drive Navigator is a device fitted in a systems slot that allows you to jump to a distant system. The specific JDN equipped determines the distance you can jump and the features available.
There are two main versions the basic Rev 1 through 4, which allow for basic navigation and the Adv version 1 through 4 that ties in with your AI Passenger and Mission manifest allowing the AI to proffer automated routes for Missions and Passengers destinations you may have active at that time.