In printmaking, an edition is a number of prints struck from only one plate, usually in a limited time frame. ... This may be a limited edition, with a pre-set number of impressions produced on the understanding that no further impressions (copies) will be produced later, or an open edition limited only by the total number that can be sold or produced before the plate wears. Most modern artists produce limited editions, usually signed by the artist in pencil, then numbered as say 42/100 to show the unique number of that impression and the total edition size.
Source:
http://en.wikipedia.org/wiki/Edition