This is the 2058th most frequent Russian word.
денис
"Денис" is a Russian male given name, equivalent to "Denis" in English.
Here, 'Денис' is used as a proper noun, designating a person's name, serving as the subject of the sentence.
Денис пошёл в магазин.
Denis went to the store.
In this sentence, 'Денисе' is the prepositional case of the proper noun 'Денис', used because the verb 'говоришь о' requires this case for its object.
Ты говоришь о нашем друге Денисе?
Are you talking about our friend Denis?
Here, 'Дениса' is the genitive case of the name 'Денис', used in a negative construct indicating the absence of Denis at the meeting.