What is cryonics?
Cryonics is the practice of cold preserving humans (or other living things) in the hopes of reviving (reanimating) them in the future.
The overall idea is to take someone who has recently died (or even is near death, assuming there is no chance of healing) and preserving them at supercool temperatures (most commonly using liquid nitrogen). The hope is that at some point in the future, medicine and technology will advance to such a degree that this person can be brought back to life and additionally, that whatever killed them can be reversed/cured.