What does the principle of the constancy of lightspeed tell us about the speed of light?

The speed of light is not relative, it is always c no matter how fast or in what way the observer is moving

Explanation:

The speed of light. In a vacuum, it is by definition a universal constant of value 299,792,458 m / s (usually close to 3 · 108 m / s), or what is the same 9.46 · 1015 m / year; The second figure is used to define the interval called the light year. It is symbolized by the letter c, from the Latin celéritās (in Spanish celerity or rapidity), and is also known as the Einstein constant. [Citation needed] The value of the speed of light in a vacuum was officially included in the System International Units as a constant on October 21, 1983, thus passing the meter to be a unit derived from this constant. The speed through a medium other than the "vacuum" depends on its electrical permittivity, its magnetic permeability, and other electromagnetic characteristics. In material media, this speed is lower than "c" and is encoded in the index of refraction. In more subtle vacuum modifications, such as curved spaces, Casimir effect, thermal populations or presence of external fields, the speed of light depends on the energy density of that vacuum.