I read a blog post about cognitive impairment. But I'm still not clear - what is it?

Problem thinking. If a person cannot utilize memory, learning, speech, reasoning, judgement, orientation, self-awareness, or appropriate social behavior, they are manifesting elements of cognitive impairment. The causes of this are truly diverse, including dehydration, drug use (prescription or "recreational"), fever, infection, brain trauma, stroke, metabolic and hormonal causes, degenerative disease.