Time dilation is a concept from Einstein's theory of relativity that describes how time can pass at different rates depending on an object's speed or gravitational field. For example, a clock moving at a high speed will tick more slowly compared to a stationary clock. This effect becomes significant as the speed approaches that of light. Additionally, time can also be affected by gravity. Clocks located in stronger gravitational fields, like those on the surface of the Earth, will run slower than clocks in weaker fields, such as those in space. This means that time is not a constant and can vary based on conditions.