Forget everything you know about relativity for a moment. Assume that there is some absolute frame of reference in the universe. That the subatomic particles and waves we are made of are themselves made out of pieces on a 3D chessboard.
Things that keep "absolutely" still stay on the same squares (well, cubes). Things that move will jump from square to square along the chessboard.
This begs the question: does light travel at a constant speed with relation to the chessboard (like sound does, relative to the air it travels in), or with relation to the source that generated it (something like ejected particles)?
The latter can be easily shown to be false by timing how long it takes light to reach you given a moving light source. What about the former? Well, on a fast-moving spaceship, you would expect light to travel much faster going towards the rear of the ship than towards the front. But this isn't the case. No matter how fast you are travelling, light seems to move the same speed no matter who creates it or which direction it is sent in.
How can this be? Most people believe there is no "3D chessboard" and that it is simply a physical law that the speed of light in all directions be the same for all observers. Personally, I believe that the "3D chessboard" does exist in some form (something that admits curvature), but we can't observe any differences in the speed of light, because our clocks and rulers are implicitly calibrated by the very same speed of light! It is like trying to measure inflation by seeing how many \$10 notes it takes to buy a \$50 note.
I will present an oversimplified picture of time dilation to make my point. The physical processes that make a clock tick depend on the speed of light. If light travelled slower, the clock would also tick more slowly, so it would be undetectable! Actually the reality is more complicated, because light would not be uniformly slower but rather slower in certain directions and faster in others, so lengths in various directions get distorted, as well as the time it takes for signals to reach an observer. But everything cancels out in the end, meaning that the observer cannot observe her own speed relative to the "3D chessboard".
If you go down this path it is very important to distinguish between distance and time as measured by us, and "absolute distance" and "absolute time" as measured against the absolute frame of reference. The equations of Relativity deal with distance and time as measured by us. "Absolute distance" and "absolute time" are things we cannot measure and do not have any units for.
Maybe there is no absolute frame of reference (I'm not aware of any evidence supporting it). But even so, I think it's a useful stepping stone to getting your head around Einsteinian Relativity.