The black cross is the arms of the State of Köln to which the town historically belonged. The keys in the first quarter symbolise St. Peter, patron saint of the State of Köln. The bishop in the second quarter is St. Benedict, patron saint of Heerdt.
Own work 
This image is taken from www.ngw.nl – Heraldry of the World –  an international civic heraldry site by Ralf Hartemink. ngw.nl/heraldrywiki/index.php?title=Heerdt
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