I've studied something so I'm able to explain better what I'm thinking about.
't Hooft duality states:
In a gauge theory with fields in the adjoint representation in presence of a mass gap only the following alternatives are allowed:
1 Magnetic condensation and electric confinement
2 Electric condensation and magnetic confinement.
I've also learnt that the object of electric nature are the ones that are affected by gauge transformation while the magnetic ones are not. Moreover, but I'm not so sure of that, magnetic objects in an electric theory can be admitted only as singularities (such as Dirac delta) in analogy to the Meissner effect in which the magnetic field can penetrate a superconductor only via tiny tubes.
So I would appreciate if someone would give me a better insight of this characterization of electric and magnetic objects since they have been presented to me only as facts to accept.
Thank you in advance.
The previous, less clear, version of my question was:
Approaching the study of QCD, I've read about the 't Hooft duality that is similar to the Meissner effect for superconductors: magnetic condensation implies electric confinement and vice versa. At first I'm not sure about what is magnetic and what is electric in QCD. Then I would appreciate if someone could give me a formal description of this phenomenon, cause I've only found this naive description. Thanks.