The notation is molecular term symbols and, the wikipedia article does seem to provide what you are asking for. The larger subject is molecular spectroscopy.
Lets look at the term on the right. The capitalized sigma refers to the angular momentum of this orbital along the molecular z axis, in this case it is zero. A capital pi would mean it is 1. Where atomic orbitals are labeled s, p, d etc., these are labeled Sigma, Pi, Delta etc. The "3" preceding it should be shifted up (as it is in the article), it indicates that this is a triplet. The g (for gerade, even) means it has inversion symmetry, the alternative is u (ungerade or odd) for antisymmetry under inversion.
You might find an introduction to the topic in the textbook for an undergraduate level course in physical chemistry.