Definition of Gibbs free energy

[Lyndon_Bui_3J]

I understand the calculations involved in Gibbs free energy but what is it conceptually?

[Hannah_Chao_1D]

Gibbs free energy is the maximum nonexpansion work (inc. electrical and mechanical work) that the system can do at constant temperature and pressure. In other words, it is the measure of energy that is free to do the maximum nonexpansion work.