When Einstein stated that "time is relative," he was referring to the concept of time dilation in his theory of relativity. According to the theory, the passage of time is not absolute and universal but instead depends on the relative motion between observers and the strength of gravitational fields.
In the special theory of relativity, Einstein introduced the idea of time dilation, which states that time can appear to pass differently for observers in relative motion. If two observers are moving relative to each other at a significant fraction of the speed of light, they will perceive time to be passing more slowly for the other observer. This effect is known as time dilation and has been experimentally verified.
In the general theory of relativity, Einstein extended this idea to include the effects of gravity. In the presence of a gravitational field, time also becomes relative. Clocks in stronger gravitational fields run slower compared to clocks in weaker gravitational fields. This phenomenon is known as gravitational time dilation and has been observed in various experiments, including the famous Pound-Rebka experiment.
In essence, Einstein's statement that "time is relative" means that the measurement of time can vary for different observers depending on their relative motion or the strength of the gravitational field they experience. It signifies a departure from the classical notion of time as an absolute and universally consistent quantity.