According to the theory of relativity, specifically the theory of special relativity, time dilation occurs when objects move relative to each other at speeds close to the speed of light.
When an object is moving relative to an observer at high speeds, time appears to pass more slowly for that object compared to the observer's own frame of reference. This effect is known as time dilation. The faster the object moves, the more pronounced the time dilation becomes.
For example, let's consider a scenario where there are two observers: Observer A and Observer B. Observer A remains stationary while Observer B travels away from A at a high velocity close to the speed of light. From Observer A's perspective, time for Observer B would appear to be passing more slowly. Similarly, from Observer B's perspective, time for Observer A would appear to be passing more slowly. This phenomenon is referred to as time dilation.
This effect has been observed and verified in various experiments and is an essential component of the theory of special relativity. It means that time is not an absolute quantity but is dependent on the relative motion between observers.