What is this HOMO-LOMO electron transition?

HOMO represents the highest occupied molecular orbital and LUMO represents lowest unoccupied molecular orbital.Electronic transition occurs when they are close in energy(orbital).
HOMO and LUMO orbitals are a pair and they lie very close together in energy of two molecules. Therefore, the interaction between them is very strong.
When we are dealing with interacting molecular orbitals, the two that interact are generally

The highest energy occupied molecular orbital (HOMO) of one molecule,The lowest energy unoccupied molecular orbital (LUMO) of the other molecule.

EX-interaction between acid and base
;NH3 +BH3
In this case the HOMO of NH3(excess electron-base) will interact with LUMO of BH3(electron deficient-base)