What is the purpose +auto.master in /etc/auto.master?

I looked into the default /etc/auto.master and I saw the following.

#
# Include central master map if it can be found using
# nsswitch sources.
#
# Note that if there are entries for /net or /misc (as
# above) in the included master map any keys that are the
# same will not be seen as the first read key seen takes
# precedence.
#
+auto.master

May I know what does +auto.master means and why is it there?