I can explain this better. If you are in a car going 55mph due north, you have a velocity that is 55mph north. if you change your direction and go 50mph north and 5mph east, you are still going 55mph, but you are only going 50mph north and now, 5mph east.

Now we will come back to why I told you this. Common experience tells us that if we are in a car travelling 80 mph, then something that is travelling 100mph will only look as if travelling 20 mph to us. If we were moving 80mph in the opposite direction as we passed it, it would look as if travelling 180mph to us. We are all aware that light travels 300,000Km/s. What if we had a spaceship that could travel 250,000Km/s and we raced a light beam, what would we see? It turns out, no matter what, light always apppears to travel at 300,000km/s, even if we are moving at 299,999km/s, it would still appear to us to be racing off at 300,000km/s. It appears no slower, or faster. This has been confirmed time and time again in experiment and observation. Why? Einstein gave his reasoning for this in his theory of special relativity. Each observer has his own clock and yardstick. Each observer is just as correct as the other. The person that is stationary will measure light moving at 300,000km/s, and the person moving at 250,000km/s will also see the light as moving 300,000km/s relative to him because his yardstick will be different then the stationary observer because of his velocity. It turns out that the speed of light is one of the few constants in the universe that all can agree too. It is also the "speed-limit" of which we can travel though spacetime.

But wait, why does time slow for those that are moving faster through spacetime? Read again what I wrote at the beginning. Then look out the window at a parked car. Ignore the rotation and revolutions of the earth or sun and spinning of the galaxy, imagine it is perfectly still. It is moving through time at light speed, the maximum speed you can move through space is also the maximum you can move through time. Imagine that car goes and now moves 100kph down the road. It is now going 100kph through space, and 299,900kph through time. As you increase your velocity through space, you decrease your velocity through time. Exactly the same as if you were travelling at a constant speed in a car north, and you changed your velocity to the east a bit. you would not be travelling as fast to the north, and you would be travelling to the east more now, but you would still be going at the same velocity overall.