Abstract: In 1905, inspired by the Michelson-Morley experiment, Einstein proposed the hypothesis of the invariance of the light speed (ILS), from which he theoretically deduced the Lorentz transformation, established the theory of special relativity (SR), and revealed the relativistic phenomena of spacetime and matter motion. However, to this day we still do not know exactly what role light plays in Einstein's SR, do not fully understand why the speed of light is invariant in the Michelson-Morley experiment, and do not truly comprehend why spacetime and matter motion exhibit relativistic phenomena. Here, in light of the locality of the physical world, we develop the principle of observational locality (POL), and propose the hypothesis of the observational limit (HOL). By taking the POL and the HOL as prerequisites, we logically deduce the invariance of observation medium speeds (IOMSs) and theoretically show that the Michelson-Morley experiment does not truly imply the ILS but demonstrates a significant phenomenon in physical observation:the speeds at which observation media transmit the information of observed objects exhibit observational invariance. In fact, the ILS is only a special case of the IOMSs, and is only valid if light acts as the observation medium; the speed of light is not truly an invariant that cannot be exceeded. From the IOMSs, we logically and theoretically deduce the general Lorentz transformation (GLT), which generalizes and unifies the Galilean transformation (GT) and the Lorentz transformation (LT). Under Bohr's correspondence principle, the GLT strictly corresponds to both the GT and the LT. On the basis of the GLT, following Einstein's logic way in SR, we establish the theory of observational relativity (OR), which generalizes SR. The theory of OR sheds light on the essence and root of relativistic phenomena:these phenomena all represent observational effects rooted in observational locality rather than real natural phenomena or physical reality. This study may provide an insight into Einstein's theory of relativity and the relativistic phenomena of spacetime and matter motion.