"Cognition" is a fancy word that mental health professionals use to describe the wide range of brain-based behaviors that we rely on every day. Cognition encompasses lots of different skills, including perception (taking in information from our sensory organs), memory, learning, judgment, abstract reasoning (thinking about things that aren't directly in front of us), problem solving, using language, and planning.
We take many of these cognitive skills for granted as we go about our routine activities. For instance, eating breakfast in the morning is a relatively complex task that involves multiple steps. First, we need to be aware of (health care professionals call this "oriented to") the time, and realize that it is appropriate to have an early meal. Next, we need to decide what to eat, which involves generating different meal choices and making a selection. Then, we need to follow the correct steps in order to prepare the meal. Even something simple like a bowl of oatmeal can be ruined if the preparation steps are not followed in the correct order (e.g., if you forget to add the water to instant oatmeal before heating it up in the microwave). Finally, we need to remember how to use utensils and swallow in order to eat.
Damage to any part of the brain can cause a cognitive disorder, which is a "catch all" term used to describe impairment in any one (or all) of the thinking skills that we described above. Cognitive disorders used to be called "organic mental syndromes" or "organic mental disorders" to indicate that these disorders had a brain or biological basis. However, the term "organic" is no longer used because it implies that all other mental disorders (not categorized as organic) do not have a biological basis. Most mental health professionals now believe that the majority of mental disorders are caused or influenced by brain chemistry or another medical issue that affects how the brain functions.