First, let's start with the old notion of relativity that Galileo and Newton came up with a long time ago.
These observations lead Galileo and Newton to formulate the principle of relativity.
The laws of mechanics are the same for all observers moving at constant velocity.
We will see the laws of mechanics, briefly, in the next week. We will not discuss them very much since these laws have changed a bit since Galileo and Newton. But the idea was that the laws of mechanics were telling you how you move when a force (a push or pull) is exerted on you. Since Newton and Galileo observed (experimentally) that objects seem to move with no push or pull when they are accelerated, they came up with this principle of relativity that the laws of mechanics only work for observers moving at constant velocity.
This is a powerful statement. It says that the laws of mechanics are the same everywhere in the Universe (independent of location). They were the same in the past and will be the same in the future. They only seem to fail when an object is not moving at a constant velocity.
With the work of Newton, we have figured out "how" the laws fail when an observer is accelerating, and we can "correct" Newton's law so that it still works. The point is that the laws of physics are unchanged if the observer moves at a constant velocity but are changed if the observer accelerate.
Galileo and Newton were pretty pleased with themselves. But then came a new theory, the theory of electricity, discovered by many people and synthesized by James Clerk Maxwell. This complicated mechanical laws, if you can believe it.
We will discuss the theory of electricity later on, but, for now, there is only one thing we need to know: the theory of electricity predicts that light is a wave moving with a speed of
the symbol "c" always means the speed of light (in a vacuum).