The Meissner state is how the superconductor reacts when a magnetic field is applied: currents of superconducting electrons expel the magnetic field from the sample. When the magnetic field becomes too strong, the system becomes a metal again. The superconductors which follow this “all or nothing” behaviour are called type I superconductors; this is the case for lead and mercury, for instance.
However, some other superconductors manage to find a more subtle compromise: they are called type II superconductors, such as NbSn alloy or cuprates, for instance. Under a small magnetic field (with a weak magnet or a magnet far from the sample), they react like type I superconductors and completely expel the magnetic field. But when the magnetic field is stronger, they prefer to adopt a compromise situation and allow some of the magnetic field to penetrate along “vortices”.
These vortices are like tubes that go through the sample from side to side and in which the material is no longer superconducting since the magnetic field can penetrate it. The material then becomes a sieve. In order to enable this magnetic field to go through the vortex, the material develops superconducting currents circulating around this pillar in a spiral motion justifying the name, “vortex”.
