MWI is a theory of quantum physics based on Schrödinger's cat, a thought experiment in which a cat is in a box with a flask of poison. A machine containing a radioactive element is set to shatter the flask in the event that the radioactive element decays. If that happens, the cat dies. That radioactive decay is a purely quantum process. Until it's observed, it has both decayed and not decayed so the cat is simultaneously alive and dead until we open the box.