In most cases, speed is described relative to the Earth's surface. For most applications of classical mechanics, this works fine, but if you examine things more fundamentally, you find that this description of speed isn't perfect. The idea that you can only describe speed relative to another object is fundamental to the theory of special relativity. A common description of this idea goes kind of like this:
You're floating through deep space at an unknown speed, with nothing in sight but distant stars. In the distance, you see another person come into view, getting closer to you. If you try to answer the question: "how fast is the person traveling?" you'll find you have a hard time doing this. Maybe you're perfectly stationary, in which case, you could measure their speed rather simply. However, if you don't know how fast you're traveling, maybe the other person is stationary and it's you who is flying through space; or you could both be moving. From all we know about physics today, it seems to be impossible to find an answer for this.
Therefore, Einstein concluded that the only way to describe speed is relative to another object. So far as we know, the only "speed" that the person floating through space has, is a speed relative to you (or relative to another reference point). There's no way to tell whether one of you is "stationary."
For things happening on earth, we've always used the Earth's surface as a reference point, and this becomes second nature. However, if you fly away from the Earth in a space ship, how do you measure your speed? If you measure how fast you're going relative to the earth, you get a different speed relative to mars, or relative to the sun, or whatever you compare it to. Just the same, your speed relative to the sun is different than your speed relative to a neighboring star, or the center of the galaxy. There's no definitive reference point that allows you to declare an absolute speed.