No lens can produce a perfectly sharp image.
This means that there will be a range of focus settings for the lens, which will give no significant improvement over an exact setting. Conversely, for a given distance setting, there will be a range of lens to subject distances over which focusing will be adequate.
This range is called the depth of field.
The depth of field of a lens improves as the aperture of the lens is made smaller, although maximum depth of field is not the same as maximum optical resolution.
The trick, in photography, is to choose the aperture so that the depth of field embraces the whole depth of the subject from the furthest to the nearest point. Good lenses are provided with depth of field markings to assist in this matter, and good SLR cameras have a 'depth-of-field preview' button, which closes the lens down to the aperture that will be used when the picture is taken.