Basically, many-worlds proposes the idea that the quantum system doesn't actually decide. Rather, that at every junction where large everyday stuff interacts with the quantum system, the timeline of history splits and both possibilities happen on different alternate branches.

In the 'Many Worlds Interpretation of Quantum Mechanics,' the trajectory of your life is no longer just one straight path to an eventuality, but is instead one path of many, on an ever-branching tree of possibilities.

Many-worlds interpretation David Deutsch argues that the way to understand the double-slit experiment is that in each universe the particle travels through a specific slit, but its motion is affected by the interference with particles in other universes. This creates the observable fringes.

The Copenhagen interpretation introduced the concept of wave function collapse but failed to precisely define the conditions that cause a collapse or why it collapses. The Everett interpretation (many-worlds) simply removes the idea of collapse and the all the unresolved questions regarding collapse.