Checker is one of Auryn's abstract base classes. Any descendant of it is a “checker”. That mean it is an object that performs minimalistic online run-monitoring of your simulation and stops the simulation if something goes wrong. Checkers are called last during the Auryn duty cycle, after the Monitor objects, and can terminate a run in any time step if the run function of the System class is called with the option checking==true (that's the default). When a checker triggers, it might still take MINDELAY time until all other ranks are notified of this run termination, in case a Checker only triggers locally on one rank, which can happen.
The most standard Checker which should be included at least once in every Auryn simulation is the RateChecker.