An eyewitness is an observer who's seen something clearly enough to describe it. If you're an eyewitness to a car accident, the police might want to ask you exactly what you saw.
An eyewitness is valuable to crime investigators because she has seen the incident happen with her own eyes. You could also be an eyewitness to something less serious, like a boy being mean to his younger brother when he thinks no one is watching, or a woman throwing her soda can on the ground. While a witness may have valuable information about something, an eyewitness is even more important because he has actually seen — or witnessed — the event in question.