Time dilation is the process of slowing down or speeding up time relative to a particular observer, typically in the context of general relativity.
For example, if you're on a spaceship traveling near the speed of light, time would appear to pass normally for you and your crew, but for someone on Earth, time would appear to be passing at a different rate.
This can be useful for various purposes, such as:
But be warned, time dilation can be hazardous to your mental health.