An inertial reference frame is one in which a free particle moves with constant velocity. Two inertial frames can move at constant velocity relative to one another without either being physically privileged.

Special relativity is built from two principles:

1. The laws of physics have the same form in every inertial frame.
2. Light in vacuum has the same speed $c$ for every inertial observer, independent of the motion of the source.

Why Galilean transformations fail

In ordinary low-speed intuition, velocities simply add. If that rule were exact, different inertial observers would generally measure different speeds for the same light pulse.

Special relativity instead changes how space and time coordinates between frames are related.

Events

An event is something assigned both a position and a time. Different inertial observers can assign different coordinates $(x,t)$ to the same event, while the physical event itself is the same.

Relativity does not say that every statement is subjective. It identifies which quantities depend on the observer and which combinations remain invariant across inertial frames.