Guide : what is the purpose of the gr_vmcircbuf.cc file and why it has been checked during the confi

Hi everyone ,
I apologise in advance if i have asked a very insignificant and
illogical
question , I am trying to understand the gnuradio programming
architecture
so as i can make my own module/block so as i can implement and do some
experiment on my concepts of dsp using gnuradio.Please someone guide me
why
the
gr_vmcircbuf.cc is been checked in the configure.ac like

AC_CONFIG_SRCDIR([gnuradio-core/src/lib/runtime/gr_vmcircbuf.cc])

in the top level configure.ac file.What it really is done in the
gnuradio in
the gnuradio.

Hope someone will be willing to teach me / guide me some of the problems
coming from my side.
Thanks in advance.