This indicates that of an edition of 250 this is the 12th impression. In this example, the number 250 most likely does not represent the entire edition. In this example that would mean the total edition is 275. The opposite of limited edition is open edition. This refers to a poster or other reproduction where there is an undefined number of the image.
Posters are open editions.
A limited edition is typically signified by a fraction in the lower margin of the image. By contrast, an Open Edition will not have a a fraction since there is no limit to the number of the image that will be printed. As a result of this, an Open Edition print is considerably less expensive than a Limited Edition. The sky is the limit with medium and substratum. Substratum are typically canvas or paper, but here again it is nearly limitless.