Albert Einstein came up with this theory to resolve certain theoretical problems in
Maxwell's
electrodynamics, but it was quickly seen to be a much more general
theory.
It is generally interpreted by physicists as a statement that any acceptable law of physics must respect certain requirements. Mathematically this is stated as "All laws of physics must be invariant under a Lorentz transformation". In English this means that no causal influence can travel faster than the speed of light.
The name relativity is a bit unfortunate, since it does not mean "all is relative", but rather, "all is related in a particular way". This confusion causes a lot of what Richard P. Feynman calls, "cocktail party philosophy".