[Discuss-gnuradio] what does the "corresponds to DC in the IF"mean?

[intermilan]

hi all:
      I recently see the "corresponds to DC in the IF" in the function set_freq() of the db_basic.cc.so what does this  sentence mean? In other word, I want to know what is the function of the parameter baseband_freq in the set_freq().I hope someone can help me the figure it out.

                                                                                                                                                                               Thank you in advance