A light clock consists of two opposing mirrors between which a photon is reflected back and forth. Depending on whether the light clock is at rest ($$v = 0$$) or moving at the velocity $$v$$, the photon must travel a different distance. In case of a stationary light clock it covers the distance $$L$$ and in case of a moving light clock the distance $$L'$$. Since the photon always travels at the speed of light $$c$$, the photon takes different amounts of time to travel the distance $$L$$ or $$L'$$.