My textbook says that in a galvanic cell, it's not possible to measure single electrode potential independently. Instead, a Standard Hydrogen Electrode is used as the system is under equilibrium. Can someone explain this concept more clearly?
Electrodes transport electrons produced from one half-cell to another, thus producing an electric charge. You need to have a reference value to compare it to in order to obtain the relative potential. Relative to the Standard Hydrogen Electrode (SHE), that is.
The SHE has a potential that is set arbitrarily to zero giving you a basis with which to compare the potentials from the electrode you're testing.
For further reading, I feel this article on the Standard Hydrogen electrode is pretty descriptive.
Potential is a relative measurement. You will need to set a reference for measuring any potential.
As in this case, Hydrogen electrode is set as the reference.
All other potentials are measured against Standard Hydrogen electrode to get their potentials under standard conditions.