What does "%if 0%{?..." mean

[Jonathan Ryshpan]

I have a spec file with this text.
        %if 0%{?build_jack_svn}
        %define jack_revision 3190
        %define jack_version 0.116.1
        %define jack_svndate 20081206.171018
        %else
        %define jack_version 0.116.1
        %endif

What does all this do?  I can't find the form "%if 0%{?mac}" defined
anywhere.  I assume it means the macro mac exists and has the value 0 
-- is this correct?

Thanks - jon