The Meissner effect is when a magnetic field is pushed out of a superconductor when it becomes superconducting. If you were to put a superconductor in the field of a big magnet and you looked just inside the superconductor, you would see that the magnetic field was much smaller than it was outside. The deeper in you looked, the closer it would be to zero. This is one of the ways that superconductors are not the same as perfect conductors, which do let magnetic fields pass through them.