When we say that time stops at the speed of light, it refers to the perspective of an object traveling at the speed of light. According to the theory of relativity, for an object with mass, time dilation occurs as it approaches the speed of light, meaning that time appears to slow down relative to a stationary observer. However, for massless particles such as photons (particles of light), the concept of time as we understand it does not apply in the same way.
From the perspective of a photon, which travels at the speed of light, its journey is instantaneous. It does not experience time passing because, according to relativity, time dilation becomes infinite for an object traveling at the speed of light. Therefore, a photon can be emitted from a distant star and reach our eyes in what appears to be zero time from its frame of reference.
However, from the perspective of an observer, light still takes time to travel from one point to another. When we observe light traveling across vast distances, we measure the time it takes for the light to reach us based on our frame of reference. This is why we can talk about light-year as a unit of distance—the distance light travels in one year.
It's important to note that while light appears to move instantaneously from its perspective, it still obeys the laws of physics and follows a path through spacetime, which may take time from the perspective of an observer.