This term is usually given as a percentage to describe the percent of malt sugar that is converted by the yeast strain to ethanol and CO2. Most yeast strains attenuate in the range of 65 - 80%. More specifically, this range is the "Apparent" attenuation. The apparent attenuation is determined by comparing the Original and Final gravities of the beer. A 1.040 OG that ferments to a 1.010 FG would have an apparent attenuation of 75%.
(From FG = OG - (OG x %) => % att. = (OG-FG)/OG)
The "Real" attenuation is less. Pure ethanol has a gravity of about 0.800. If you had a 1.040 OG beer and got 100% real attenuation, the resulting specific gravity would be about 0.991 (corresponding to about 5% alcohol by weight). The apparent attenuation of this beer would be 122%. The apparent attenuation of a yeast strain will vary depending on the types of sugars in the wort that the yeast is fermenting. Thus the number quoted for a particular yeast is an average. For purposes of discussion, apparent attenuation is ranked as low, medium, and high by the following percentages:
65-70% = Low 71-75% = Medium 76-80% = High