The PopulationRateMonitor records the firing rate of a neuronal population or a SpikingGroup in Auryn. A typical use case is the following

sprintf(strbuf, "%s/%s.%d.prate", dir.c_str(), file_prefix, world.rank());
PopulationRateMonitor * pmon_e = new PopulationRateMonitor( neurons_e, strbuf, 1.0 );

here the file name of the output file of the current rank is first stored in the variable strbuf (which has to be declared at an earlier point), then the PopulationRateMonitor pmon_e is initialized for the excitatory NeuronGroup neurons_e. The the third argument 1.0 tells the constructor to average the population firing rate over one second.