Deep space is an almost perfect vacuum. Unlike planet earth, with its gas filled atmosphere, there is almost no air in space, and no particles floating about for light to interact with.
When light travels through empty space, it doesn't interact with anything. It simply goes straight ahead without bouncing off anything. Therefore, nothing stops the light from reaching Earth.
Since no matter is present to stop the light, it continues on its way unimpeded. At this point, the light is called "free" or "unbounded." Unbounded light is invisible to us, since it never interacts with anything. We call unbounded light that which travels in free space.