Collective memory is the equivalent of the genome in genetics. In his Theses on Feuerbach Marx called it the ensemble of the social relations. It goes back to mytochondrial Eve. It is how we have inherited consciousness and language from all previous generations. It could be pristine and truthful if it was not manipulated by private interests, class. The distortion of consciousness, largely through the media produces ideology. Ideology is collective memory in the service of only a segment of society that today we call one percent.

Collective memory means different things to people in different disciplines and even within a discipline. Do you simply mean how media make "memories" (another word with different meanings) people have more similar than they would be without the media?

For a fairly general response, 'collective memory' is the way societies or groups of people remember an event. The media's role in this remembering is that it provides a particular narrative of the event that may be inscribed in the collective consciousness. Who and what is remembered and which narratives are then passed down is thus heavily influenced by the media (but not always controlled by the media). The media today contains different and often competing voices on how an event is narrated. Social media, films, documentaries, news channels, newspapers etc. contain different voices and perspectives on a given event. An interesting aspect of this process is an examination of which stories are being told and which are left silent. 

Relates to the influence of media like television inscribing memories of past events into the collective consciousness of groups or societies by utilizing effective storytelling, narration and visual aids.