What does eye power -6 means and how close to blindness is it?

One of my friends went to an eye specialist for checkup and the doctor told him that his eye power changed from -6 from -4.5. He is really very upset and doesn’t want to discuss anything on this. I can understand that -6 means the vision is in very poor state but don’t have any idea how much worse it is.

1. Could someone explain me how what eye power -6 means in respect to closeness of blindness?
2. What can be the minimum negative eye power and maximum positive eye power of an eye?