The really short take is this: toning "guilds" the silver in your print
with another substance.
Almost any other substance is less susceptible to deterioration than
silver (how often did your mother have to polish the silverware? Every
time it was to be used, right?) so the print is more archival if the
silver is treated. Selenium is often used when the appearance of the
print should remain the same but a protective treatment is desired. With
some papers selenium will also make a substantial visual effect. Gold
toning changes color and greatly enhances permanence. Sepia toning is
usually done primarily for the visual effect but I think it has some
slight beneficial effect for permanence. You can even tone with
platinum, for enormous archival effect, but it would be easier and
cheaper to make a platinum print in the first place.---Carl