Cognition is a term referring to the mental processes involved in gaining knowledge and comprehension, including thinking, knowing, remembering, judging and problem-solving. These are higher-level functions of the brain and encompass language, imagination, perception and planning. (Defined by Psychology at about.com)
The Cognitive effect model found that the media has an indirect influence on an audiences' attitude. A viewer already has set ideas and opinions, the media cannot do much to change those. However by showing certain stories more often than others and shaping the agenda they can shape what an audience puts importance on. For example, if the media reports more on the economy than on international news, then people will have more information on the economy and think that the issue is more important than other things that are going on around the world.