"Archival" prints from "wet" chemical darkrooms of the past are processed and washed [and washed and washed!] in a special manner to remove all the chemicals and light sensitive particles that would lead to the deterioration of the paper print. Through the use of different techniques, the photographs were able to be preserved, theoritically if the processes were done properly, for hundreds of years. It is a time intensive process that needs to be done just right! Color prints, slides and tranparencies from those days of analogue film were never considered to be archival.
In today's digital world, "archival" means the same thing, that through the use of special papers and inks, images will be preserved for hundreds of years [at this point, longevity testing by such labratories as Wilhelm Imaging Research have certified longevity of between 100 and 200 years]. But now instead of using special chemical processes and long hours of washing the chemistry out of the papers, it means using special pigment based inks and nonacidic, buffered papers of the type used by artists for hundreds of years.