Any measurement of position, distance, or speed must be made with respect to a frame of reference. For examble, while you are on a train traveling at 80km/h, you might notice a person who walks past you toward the front of the train at a speed of 5 km/h. This is the person's speed with respect to the train as a frame of reference. With respect to the ground that person is moving at a speed of 80km/h + 5 km/h = 85 km/h. When specifying the motion of an object, it is important to specify not only the speed but also the direction of motion.
Frame of reference is very important in interstellar travel. Landing on the moon or Mars requires that calculations for speed and rate of closure are carefully monitored so that each result is in its correct frame of reference. You wouldn't want to try land on the moon, but only have speeds relative to Earth!
In a less abstract example, imagine two cars driving along a highway. One is in the slow lane going 50 km/h (yes very slow), and the other is in the fast lane going 100 km/h. If you were in the slow car going 50km/h the faster car would pass you at the same speed as you would pass some sitting along the side of the road!
Cool, huh?