An electrode in contact with an electrolyte solution of same ionic nature (for example; Cu electrode in CuSO4 solution, Zn electrode in ZnSO4 solution) tends to either undergo oxidation or reduction due to which there develops a charge separation creating a potential difference.
The potential difference is called the electrode potential, and when measured under standard condition of 1M electrolyte concentration, 298 K temperature, and 1 bar atmospheric pressure (if gas is involved), it is called standard electrode potential and is represented as Eo.