How to read electron energy levels abbreviations?
Those are Russel Saunders atomic term symbols and represent the quantum numbers of the many-electron wave function.

The notation scheme is $^{2S+1}L_J$ where $2S+1$ is the spin multiplicity, $L$ is the angular momentum quantum number and $J$ is the total angular momentum due to the coupling between the former two.

There are also molecular term symbols using greek instead of latin letters for the angular momentum. Those are commonly used for diatomic molecules.

You should find the Russel Saunders Coupling scheme in standard physical and quantum chemistry books. Molecular term  symbols are for example explained in detail in Molecular Spectra and Molecular Structure: I. Spectra of Diatomic Molecules by Herzberg and Huber (1950).
This link may give you a preliminary idea : http://wwwchem.uwimona.edu.jm/courses/RScoupling.html