What is the "Pre-shared keys" tab for?

From:	"Joe Lagreca" <lagreca at gmail dot com>
To:	m0n0wall at lists dot m0n0 dot ch
Subject:	What is the "Pre-shared keys" tab for?
Date:	Tue, 11 Apr 2006 21:40:13 -0700

Please excuse my ignorance, I did search the online user manual and
mailing list archive, and couldn't find an answer.

Under VPN -> IPsec settings, what is the "Pre-shared keys" tab/section
for?  Is it as simple as a list for the admin, of the Pre-shared keys
already being used?