Quantum Immortality is an idea in which it is put forward that the consciousness stays alive even though the conscious being dies. For example, someone sets off a bomb beside the victim, that victim survives in an alternate universe by being injured but living, or by the bomb not blowing up. However, in the original universe, the victim "dies" in the blast. The consciousness continues to exist in another, perhaps many alternate universes. This is related to the thought experiment of Schrödinger's cat.
The idea is that if you use a special gun that goes off, something called a quark is spinning one way, but not if it spins the other way. However, the quark somehow manages to spin both ways at once, so the universe splits into two separate possibilities as the person pulls the trigger. In one universe, the person survives, in the other, the person dies. The person themself does not notice anything different.