Refueling in Space

I was recently watching a video describing Space X's plans for a lunar landing system and one of the points raised in the video was the difficulty of refueling spacecraft in space. Thus, I was intrigued from this tidbit from an article on Space Force's proposed  RG-XX "neighborhood watch" satellites. Among other capabilities Space Force will be requiring, the satellites "will be the first Space Force satellites capable of refueling in orbit". It will be interesting to see how this technology develops.