This is the 9268th most frequent German word.
marcus
"Marcus" is not a German word but rather a proper noun, typically used as a given name.
Here 'Marcus' is a proper noun indicating a person's name and serves as the subject of the sentence.
Marcus spielt gerne Fußball.
Marcus likes playing football.
In this sentence, 'Marcus' identifies the person whom the speaker met and functions as the direct object.
Ich habe Marcus gestern getroffen.
I met Marcus yesterday.
The word 'Marcus' here is in possessive form, indicating ownership of the 'idea' being discussed.