An aspheric lens is simply a lens with a surface which is not perfectly spherical or not perfectly convex or concave, but has different areas on the lens with different degrees of curvature. By this way a single aspheric lens can often do the job of two or more spherical lenses, resulting in a lighter instrument and/or a cheaper one. An excellent example of a high quality binocular with aspherical lenses is the Minox BD 8.5×42 BR Binoculars.
However, to contribute substantially to optical quality, these lenses have to be ground (more expensive process) and not made from a mold (cheaper process), as in the case of cheaper binoculars boasting aspherical lenses. In expensive binoculars with aspheric lenses these lenses have sophisticated shapes and improve the quality of the images dramatically. These are very difficult and expensive to produce, but the results can be incredible.