What is magnetic bottle and how does it work?

Answer 1

In a Tokamak Toroidal (doughnut shaped) fusion reactor it keeps the plasma confined and away from the reactor walls - it is a force field that controls and confines superheated particles. Usually they are unstable and collapse quickly. That is fusion's main problem.

Answer 2

A magnetic 'bottle' is an arrangement of coils so that the magnetic fields they generate will oppose each other and (hopefully) contain a plasma long enough to allow a fusion reaction to be controlled. So far they have not been successful.