 The basic idea is this. After an experiment, the quantum state of the system plus apparatus is typically a superposition of terms corresponding to distinct outcomes. As the apparatus interacts with its environment, which may include observers, these systems become entangled with the apparatus and quantum system, the net result of which is a quantum state involving, for each of the possible experimental outcomes, a term in which the apparatus reading corresponds to that outcome, there are records of that outcome in the environment, observers observe that outcome, etc.. Everett proposed that each of these terms be taken to be equally real. From a God’s-eye-view, there is no unique experimental outcome, but one can also focus on a particular determinate state of one subsystem, say, the experimental apparatus, and attribute to the other systems participating in the entangled state a relative state, relative to that state of the apparatus. That is, relative to the apparatus reading ‘+’ is a state of the environment recording that result and states of observers observing that result (see the entry on  Everett’s relative-state formulation of quantum mechanics,  for more detail on Everett’s views).