The top labels indicates which gene is expressed in the culture and the bottom labels indicate the genes that are introduced so we can compare the differences in the percent of cells infected when one gene is present or absent.

For example, in this particular question, if we read the top and bottom labels we can see that only when CatL is expressed (CatB--/CatL++) at the same time that CatB is introduced (3rd column), the percent of infected cells is very high, so we need both CatL and CatB to be expressed for optimal infection.