Outer space is another way to say "the universe," or "space." If your dream is to become a NASA astronaut, then you better be comfortable with the idea of floating around in outer space.
When a spacecraft travels beyond the Earth's atmosphere, it goes into outer space, which is generally defined as all the space in between every astronomical object, including planets, moons, and stars. Astronauts dream of a chance to travel into outer space, and astrophysicists study the properties of everything that exists in outer space. Humans have always been fascinated by outer space, though they've only been able to travel there since the mid-twentieth century.