It is artwork that is printed a fixed number of times on the same material and at the same size. Every artwork in the edition is identical. There may be other editions of the same artwork printed on a different material and different size.
The amount of prints in an edition never changes. If you buy from an edition of 20, only 20 will ever be printed in that size and material. The smaller the edition, the more valuable and collectable the artwork.
You can tell what size edition you have by looking at the signature on the artwork. Usually there will be a number like 3/10 which means it is the 3rd artwork in an edition of 10.