If two observers move relative to each other, they do not always measure the same amount of time between the same events. A moving clock is observed to tick more slowly than a clock at rest, from the perspective of the other observer.

The relation is $$\Delta t = \gamma \Delta \tau$$ where $\Delta \tau$ is the proper time measured by the clock itself and $\gamma = \frac{1}{\sqrt{1 - v^2/c^2}}$. When $v$ is small compared with $c$, $\gamma$ is close to $1$.

Time dilation sounds impossible at first because our everyday experience is too slow to notice it. But at speeds near $c$, the effect is real and measurable.