Q.What did Meissner actually observe which is known as Meissner effect?

Superconductivity and the Meissner Effect

Most metals lower their electrical resistance as they cool, but the resistance never quite vanishes. In a superconductor something dramatic happens: below a sharp critical temperature $T_c$, the resistance drops abruptly to exactly zero. A current started in a superconducting loop can circulate for years with no measurable decay, because there is no resistance to dissipate it.

Zero resistance

Below $T_c$ the material carries current with no voltage drop and no heating ($P = I^2R = 0$). Mercury, the first superconductor found (by Kamerlingh Onnes in 1911), becomes superconducting near $4.2$ K. Different materials have different $T_c$; each also has a critical magnetic field and critical current above which superconductivity is destroyed.

The Meissner effect

Zero resistance alone does not define a superconductor. The deeper signature is the Meissner effect: when a material is cooled below $T_c$, it expels magnetic flux from its interior — the field inside becomes zero. The superconductor behaves as a perfect diamagnet, actively pushing out the field rather than merely trapping whatever was there before. This expulsion is what lets a magnet levitate above a superconductor.

Why it matters …