What does 75-300mm mean?

Answer 1

this is the number of millimeters a lens will make a sharp image at, usually from its back element. 50mm is usual on 35mm film cameras-i.e.as your eye sees a subject. Digital is different for each camera.

Answer 2

It means the focal length of the lens is infinitely adjustable from 75 to 300 millimeters.

And the focal length is important as it represents the field of view. A wide angle lens might have a 90 degree field of view which is wider than the average human eye and might help create an artistic effect. Or show more of an interior room. A narrow field of view, like the 300mm setting of the lens described above, has the same affect as a telescope by magnifying distant objects.