If you are driving 30 mph and throw a ball out the window with a speed that from your point of view is going 10 mph faster than your car a stationary observer would see the ball going 40 mph, the 30 mph of the car plus the 10 mph of your throw.
Now let's change the car into a rocket going 1/2 the speed of light and you shine a flashlight out the front window of the rocket you will see light moving away from you at the speed of light.
For a stationary observer you would think that the light of the flashlight would be traveling at 1 and 1/2 times the speed of light but it looks like the light is traveling at the speed of light.
This is caused because the speed of light in a vacuum is a constant regardless of the speed of the observer. What happens instead is that as you approach the speed of light the time you experience begins to slow down relativeto the time of a stationary observer.
This was actually proven by putting an atomic clock on a plane and flying around the world. When the clock on the plane was compared to an atomic clock on the ground the one on the plane was slightly off.
Anyway this has been me trying to explain relativity to the mangadex comment section
TL
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The speed of light is a constant regardless of your speed.