What is the apache2ctl "-k" flag?

Yeah, it's a little buried in the description:

When acting in pass-through mode, apachectl can take all the arguments available for the httpd binary.

apachectl [ httpd-argument ]

So let's look at http's documentation then:

-k start|restart|graceful|stop|graceful-stop

Signals httpd to start, restart, or stop.

So if you use -k <option>, you'll simply pass on to httpd, which needs this argument.

If you don't use the -k, apache2ctl will instead look for commands that it will handle itself, which are again the same as httpd would take.

Looking at the source code exhibits this behavior, where a case statement checks whether the first argument is one of the recognized internal commands, and finally (as a fallback), everything's passed onto httpd.

case $ARGV in
start)
  HTTPD ${APACHE_ARGUMENTS} -k $ARGV # <= note the -k here
  # ...
stop|graceful-stop)
  # ...
# ...
*)
    $HTTPD ${APACHE_ARGUMENTS} $ARGV
    ERROR=$?
esac

Edit to add: Sorry, slhck types faster than me :D

'apache2ctl' is actually just a front-end for the 'httpd' executable and runs in two modes depending on if you're wanting it to be SysV init scriptable or if you're wanting to pass-through options to the httpd executable. The -k actually gets passed through to httpd.

http://httpd.apache.org/docs/2.2/programs/apachectl.html

When acting in pass-through mode, apachectl can take all the arguments available for the httpd binary.

apachectl [ httpd-argument ]

So from the httpd man page, http://httpd.apache.org/docs/2.2/programs/httpd.html

-k start|restart|graceful|stop|graceful-stop Signals httpd to start, restart, or stop.