A time crystal is a repeating pattern in the time dimensions which breaks symmetry with the law of conservation of energy. Besides the energy used to set up the experiment, the repetitions appear to require no other energy to loop forever.
See the video of the first recorded space-time crystal.
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In a recent experiment, researchers were able to link two time crystals together by placing them close enough that they could influence each other. The result was a combined time crystal that can represent two states, like binary. This is important because it means it’s feasible to interact with time crystals allowing us to create more complex arrangements. For example, someday this could be used to create a new kind of quantum computer.
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The theory of relativity shows that time is not a constant, but a relative measurement to the observer. For example, it’s long been established that clocks running further away from the Earth’s gravity run slightly slower than those on the surface when they are compared.