Why is the standard hydrogen electrode taken as zero potential?

We can never measure the absolute potential of a single electrode on its own, because any measurement needs a complete circuit with two electrodes. So chemists agreed on a common reference and assigned the standard hydrogen electrode (SHE) a potential of exactly 0.00 volts by convention. The SHE uses hydrogen gas at 1 bar bubbled over a platinum electrode dipped in 1 molar H+ ions at 298 K. To find any other electrode's potential, you connect it to the SHE and measure the cell voltage. Because the SHE contributes zero, the reading directly gives that electrode's standard electrode potential. A positive value means that electrode is reduced more easily than hydrogen, and a negative value means less easily. It is a chosen benchmark, like measuring height above sea level.