As mentioned in a basic level article, an electrode potential arises due to the formation of an electric double layer.
To quantify the electrode potential of a metal/metal-ion system like Zn2+/Zn, we need to connect one lead of a voltmeter to the electrode and place the other lead in the electrolyte. However, such a measurement is not accurate as we are measuring the potential difference between the electrode and the solid-solution interface created by the second lead. In fact, there is no accurate way to quantify the absolute electrode potential of the electrochemical system. We can however measure the electrode potential of a system relative to another system, i.e. another half-cell, accurately.
As there are many different half-cells (e.g. Fe3+/Fe2+, Cu2+/Cu, Ag+/Ag etc.) that we can use to measure the relative potential of a particular half-cell, it is wise to select a reference half-cell for all measurements. The choice of this reference electrode falls on the standard hydrogen electrode (SHE).