In photography the sharpness (ie the crispness of the infocus and detailed parts) is affected by a variety of factors. One of those factors is the aperture you select on the lens you are using.
Most lenses (all) are not their sharpest when shot wide open (ie with the aperture blades fully open - using the smallest f number). There is variation and some are very sharp when wide open, but its still not that specific lenses sharpest point.
Sharpness will increase as the aperture is stopped down (bigger f numbers used). Typically up to around f8 the sharpness will increase with f8 being the sharpest possible aperture. (in practice this will vary from lens to lens - f8 is a good rough guide point though).
After f8 the sharpness will start to deteriorate as diffraction starts to kick in. Now diffraction is also affected by teh circle of confusion - which in practice is linked to your sensor/film size. The smaller the film/sensor the sooner diffraction will appear to be having a noticeable effect on the quality of your photos.
For a crop sensor (canon/Nikon) DSLR camera body you'll typically be able to get to around f13 and still have a good quality of sharpness to your photos. After that point the sharpness will start to drop more and more rapidly. f16 is usable - f22 and you're starting to see it f36 and you'll see softness on most lenses.
With a fullframe DSLR/35mm film you'll get to around f18 or so before you reach the same point where softness starts becoming more apparent.
However - this point of too soft will vary some depending on your editing, how you output your photos (prints, web resized images etc..) So sometimes you can use a smaller aperture and the softness from diffraction will have less to no effect on your final actual output photo.
