Sometimes confused with "flight", Interstellar Travel is a term used to describe the ability of some characters to travel through the reaches of space where no human body could probably be without the aid of technology or some other sort of protection. For humans, it is impossible to survive space therefore characters like the Vixen or Hawkman would not be able to fly in space. However, characters such as Captain Marvel or Black Adam while human in nature are enhanced to allow for space flight.
For non-humans, this ability allows their physiology to survive the harsh environment of space. Characters like Superman are immune to the nature of space and are not inhibited by it's lack of gravity.