What are host cellular factors?
Generally spoken, "host cellular factors" are proteins, receptors etc. of the host cell. In this case, they are important for the HIV infection. The authors write:


  Recently, a genome-wide siRNA analysis revealed over 250 host cellular
  factors that influence HIV-1 infection.


This means gene silencing techniques where used to identify the host cellular factors which are important for the infection, you can find these results here: "Identification of host proteins required for HIV infection through a functional genomic screen."
The overall goal is to identify proteins which are important for the function and infection of HIV to understand the interaction between virus and host cell better.