A COD ( complément d'object direct) is the direct object of a verb. That concept is the same as in English. For example:
I saw you enter the shop. -- Here "I" is the subject "saw" is the verb, and "you" is the COD.
Marie did her laundry. -- "Marie" is the subject, "did" is the verb and "the laundry" is the COD.
Google has a lot of information on this subject, too.
