APO is a designation alluding to the apochromatic characteristics of a lens. This designation addresses the ability of a lens to focus all the wave lengths of light on the same film plane (whether that be film or enlarging paper). Different colors of light will have different wave lengths. These different wave lengths would tend to focus at different distances from the lens nodal point. This would lead to departure from sharpness at high degrees of magnification.
Originally Posted by Ka
Most good quality lenses of modern design will prove to be sufficient for all but the most discriminating use. By these I would include the El Nikkor, the Schneider Componon, and the Rodenstock Rodagon.
The apochromatic lenses in each of the manufacturers product lines mentioned would be the lenses that are above the lenses that I have identified. One will normally pay substantially more for APO lenses.
An alternative to conventional enlarging lenses are lenses which were designed for the graphics field. These lenses are normally slower speed (won't open as wide). However these are normally APO lenses. There are normally a number of these listed on Ebay as well. These would normally be barrel mounted lenses as opposed to lenses mounted in shutter. If I were looking for a lens of this type, I would look to a Goerz Apo Red Dot Artar of the appropriate focal length.