The aperture number is an indication the ratio between the focal length
and the size of the effective opening of a lens.
A smaller aperture-number means larger lens opening. This also means
more light passing, shorter exposure times, less blur from moving of
camera or subject, but also a smaller range of sharpness from foreground
to background.
The effect of the latter is more noticeable for lenses with long focal
length. The old 4x6 cm camera's (with standard 100mm or so lenses) had a
very small sharpness range whilst digital cameras (with standard 12mm or
so lenses) give a much larger sharpness range from foreground to background.
Therefore the difference between 2.8 and 8 will not be very obvious with
digital cameras.
Reinder