I ran across this printing process while researching the Philadelphia Bible Society and them being the first to print a stereotype Bible in the United States in 1812. This process was already being used in England and Europe and very sparingly in the United States but not with regard to Bibles. Here is a link that tells a little about the Philadelphia Bible Society and their printing of the 1812 Bible. That link also has a few pictures of the 1812 Bible.
Wikipedia says that it is a solid plate cast in papier-mache or plaster. In other words you're making a copy of the plate to print with, then recast when your printing plates wear out. Britannica online gives a little more insight to the process. Saying that these plates were stronger than a composed plate.
This printing process is still in use today but quickly falling by the wayside with more modern processes.
Another informative site is from Old and Sold entitled The Plaster of Paris Process.