In order to understand special relativity you must remove from your mind the idea of objective velocity, or time of an event or duration or distance. By doing this, Einstein was able to formulate a consistent theory in which light would always be observed (calculated) to be travelling at the same velocity (C) by any observer. There is no point in space that can be considered still in objective terms but only still relative to an observer in the same "inertial frame", i.e. who is traveling at the same velocity.

Light (in a vacuum) is always observed to be traveling at one velocity (C) in any experiment, whether it is in your own inertial frame or in another which is moving at high speed relative to yours. This is of course counter-intuitive. In Galilean (common sense) relativity a bullet shot from the front of a fast moving train would have a velocity relative to the track that is the sum of those of the train relative to the track and the bullet relative to the train. That is not the case when a beam of light is fired from the front of a fast-moving spaceship. An observer who is still (not moving with the spaceship) would calculate the light velocity to be C, the same as the observer on the spaceship.

To resolve this apparent paradox it is necessary to make use of time dilation, distance contraction, and lastly the non-simultaneity of events separated by distance in the direction of travel in a moving inertial frame. That would include that, clocks at the front and back of a long, fast-moving spaceship, which have been synchronised by a traveller on the spaceship will not be considereded to be synchronised by an observer at rest.