When an electrode in put in solution of ions, a charge is developed between the solution and the electrode. This charge is termed a electrode potential. The electrode potential cannot be measured individually and it is measured in reference to a standard hydrogen electrode.
Standard Hydrogen Electrode
An electrode in which pure dry hydrogen gas is bubbled at 1 atm and 298K about a platinized platinum plate through a solution containing H^+ ions ( for example - HCl solution)
The emf produced is taken as zero volts. All other potential are expressed with SHE potential as zero.