A refuelling depot is a a very rudimentary base, so small it doesn't even have an Exchange for trading. They feature a small Shipyard terminal which doesn't sell ships, of course, but at least allows you to order repairs, replentish your missiles and so on, and they have a tiny bar so weary space truckers and privateers like yourself can relax for a spell. That's pretty much it.
There are only two refuelling depots in the game, XNN-26 (Halfway system) and BPD-89 (Belle system).