Well, as you all might be knowing, all the metals are also known as conductor as they allow current to pass through them easily. For example: Iron, Copper, Aluminum etc. You might have seen wires made of these metals. And when current is passed thorough them, it gets heated. This heating is because of the resistance it offers to the passing current and is known as Joule's heating. Some of the metals can carry more currents without getting heated compared to other. This is because of their inherent configuration (I will discuss some other time). Again, when the metals are cooled, the conduct even more current, which means, their resistivity towards current (electrical resistivity) decreases.
In 1911, while cooling down mercury (one of the metals) to ultra-low temperature, Kamerlingh Onnes found an entirely new behavior of it. He found that the electrical resistivity of mercury suddenly fell down to zero at a certain temperature near to 4 K. This process was reversible and was found on some more elements. This indicated a complete loss of resistivity in some of the metals at a very low temperature. This behavior of a conductor was termed as Superconductivity.