What does this lambda–sigma notation mean?
The notation refers to the valency (number of valence electrons involved in bonds, i.e., either 3 or 5) and coordination number (number of substituents attached) in organophosphorus compounds. For example, it could refer to the tautomeric forms of phosphonate esters:1



So the λ3σ3 refers to a P with 3 bonding valence electrons (the lone pair doesn't count) and 3 substituents attached while λ5σ4 refers to a P with all five of its valence electrons involved in bonds and 4 substituents attached. 



Reference


Kraszewski, A.; Stawinski, J. H-Phosphonates: Versatile synthetic precursors to biologically active phosphorus compounds. Pure Appl. Chem. 2009, 79 (12), 2217–2227. DOI: 10.1351/pac200779122217.