The Meissner effect describes the way a superconductor responds when it enters the superconducting state in an applied magnetic field. Electrical currents near its surface rearrange the field so that magnetic flux is expelled from its interior under suitable conditions. This response is a defining feature of superconductivity, not simply a consequence of having very low resistance.
It helps researchers separate a superconducting phase from a material that merely conducts electricity extremely well. A sample that shows a sharp electrical resistance drop is interesting, but a magnetic response consistent with superconductivity provides an important second line of evidence. Experiments may measure magnetization, susceptibility or magnetic shielding.
Magnetic behavior has qualifications. In type-II superconductors, flux can penetrate as vortices above a lower critical field, and trapped flux can complicate a simple expulsion measurement. The magnetic signal also depends on sample shape, temperature and measuring conditions. A claim should not be judged solely by whether a dramatic levitation photograph is available.
When reading a superconductivity paper, look for the magnetic measurement protocol, the applied field and whether the transition matches transport data. A demonstration of levitation alone is not a full characterization, and failure to see perfect expulsion under every condition does not automatically rule out superconductivity.