Cryonics is the practice of preserving a person's body, or sometimes just the brain, at very low temperatures immediately after legal death, in the hope that future medical technology will one day be able to repair the underlying damage and restore the person to life and health.
The core idea is simple: legal death is declared when the heart stops, but the information that makes you who you are, encoded in the structure of your brain, does not vanish instantly. If that structure can be preserved well enough, it may be possible to recover the person later, even if doing so is far beyond what today's medicine can manage.
Cryonics is not a claim that revival is guaranteed. It is a calculated bet that a preserved patient has a better chance than one who has been cremated or buried. To understand how the preservation actually works, see how cryopreservation works.