Great question — answering as the UphillTech team:
Can you throw a ball so that it goes 200kmh/h? Probably not. But, if you're being driven in a car that goes as fast as that and then throw a ball (ignore the air resistance -- they don't have that in space), The ball will have a speed of 200km/h plus whatever you give it. In essence, anything given a speed x will travel relatively (meaning compared to the thrower ) at that speed, but absolutely (meaning in comparison with the surrounding space ) at speed v+x, where v is a speed of a thrower. This explanation, of course, works fine with low speeds (200km/h is slow in physics). If you want to know what would happen at warp speeds... well, you'll have to wait for physicists to find that out....
Full answer by Vedran Šego: https://movies.stackexchange.com/a/14462 (CC BY-SA 3.0).
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