You will find "DLA" referenced in many of the DSLR camera reviews on the site. DLA is the acronym for Diffraction Limited Aperture. This aperture value is the result of a mathematical formula that approximates the aperture where diffraction begins to visibly negatively affect image sharpness at the pixel level.
Diffraction at the DLA is only barely visible when viewed at full-size (100%, 1 pixel = 1 pixel) on a monitor or when output to a very large print. As sensor pixel density increases, the narrowest aperture we can use to get perfectly pixel-sharp images gets wider.
DLA does not mean that narrower apertures should not be used – it is simply the point where image sharpness begins to be compromised for increased DOF and longer exposures.
And, higher resolution sensors generally continue to deliver more detail than lower resolution sensors at apertures narrower than the DLA – until the "Diffraction Cutoff Frequency" is reached. The progression from sharp the soft as the aperture narrows beyond DLA is not an abrupt one – and the change from immediately prior camera models to new models is usually not dramatic.
Check out this specific diffraction comparison example using the ISO 12233 chart comparison tool. The mouseover feature will show you the image quality degradation at f/11 by comparing to f/5.6.
Many cameras have been tested using the lens shown in that example. Learn how diffraction affects your image quality from your specific camera (or select a camera that has similar pixel density to your camera). When faced with a narrow aperture situation, you will have the knowledge to make the right decision.