Quantum immortality states essentially that from an observer's perspective, they must continue to exist in a world where they can observe. In other words, it's the observation selection effect in a quantum physics flavor.
Suppose you were playing Russian Roulette in the real world (don't do this). Suppose instead of a normal game, you played it with three chambers full, three empty, so every round is a coin flip (don't do this), spinning the cylinder in between.
Quantum immortality states you'll always get an empty chamber from your own perspective, because in those worlds where you got an occupied chamber, you ceased to observe.
Since you are still observing, you must have been in one of the universes where the chamber was empty.
Suppose every chamber was full (don't do this). You'll find that you were in a world where the bullet was a dud, or at the last minute an owl flapped in through the window and snatched the gun out of your hand, or that some event, however improbably, prevented the gun from killing you because... again.. you continue to observe.
Which sounds neat, until you really start to consider the implications of this idea.
First off, all that is required is that you continue to observe. Meaning, quality of life is not an issue. Merely that you continue to observe.
So, back to that Russian Roulette game. Maybe the gun does go off, but you sneeze at the last second, and it blows your jaw off. Now you're drinking soup through a straw for the rest of your life... but as long as you continue to observe, that's fine.
Or you get old. Really old. Super duper old. You can't eat anything but Jell-O. You've broken your hip and you're in chronic pain. You're incontinent and need to be changed three times per day. You're two hundred years old, scientists can't figure it out, but for some reason you just... don't die.
Quantum immortality is a doorway to hell. Fortunately, it's bunk. More than 100 billion people have ever lived, and the vast majority of them are dead as doornails. You're not special.