Is the speed of light the same everywhere in the universe

Is the car speed constant? I dont think so? Maybe to some things but it changes? Im getting lost. Speeds are constant? How do we know if the light speed is constant?

Please don't use very hard words so I can understand it clearly :)
Thank you in advance.

The speed of light first came from Maxwell's equations. In there, there is a term of c, which interpreted as the speed of light. However, there is no hint at any reference frame to this, which was a major problem.
Einstein comes along and suggests that the speed of light is the same for everyone, no matter how fast they're moving. The aftermath of this is general relativity. Before this, scientists were toying with the idea of the ether, a non-moving plane which light travels at the speed of light upon. All experiments so far have disproved the existence of this ether, strengthening the theory of relativity, and the speed of light being the same everywhere.

Short of it is, the speed of light's the same for everyone

Yes, the speed of light is a constant. Car speed is also a constant if it travels at light speed, which is 299,792,458 metres per second. If you can run at that speed, your running speed is a constant as well. You see, it is the speed that is a constant: not the object. The only problem is, if you move as fast as light, your whole body and everything that move together with you, will have to be reduced to a wave energy form, similar to light, photon or some form of electromagnetic radiation.

The reason why speed of 299,792,458 metres per second is a constant is because that's the fastest an object moves and can still exist. Anything that moves faster then that will have to give up their existence as light/photon/electromagnetic wave and there by can no longer be perceived as an existence. Any slower it will develop into mass object. The equivalence is described by the famous equation: E = mc2