Can someone explain this in simpler terms?
- No magnetic field lines start or stop within the volume enclosed by the surface.
Typically, the closed surface is a mathematical surface (Gaussian surface) which simply defines an 'inside' and 'outside'.
Since, as far as we know, there are no magnetic chargesmagnetic charges from which magnetic field lines start or end, any magnetic field line entering must exit through the surface; any magnetic field exiting must enter through the surface.
Thus, the number of field lines entering equals the number of field lines exiting and the flux of the magnetic field through the surface is zero.