This is the userspace part of the VirtualBox Guest Additions.
Together with virtualbox-kernel-addons they enable shared folders,
shared clipboard, etcetera, inside your Slackware guest machines.

You'll probably want to add the following lines to your rc.local:
	# Start vboxadd
	if [ -x /etc/rc.d/rc.vboxadd ]; then
	  /etc/rc.d/rc.vboxadd start
	fi

	# Start vboxvfs
	if [ -x /etc/rc.d/rc.vboxvfs ]; then
	  /etc/rc.d/rc.vboxvfs start
	fi

	# Start vboxadd-timesync
	if [ -x /etc/rc.d/rc.vboxadd-timesync ]; then
	  /etc/rc.d/rc.vboxadd-timesync start
	fi

This will load the support kernel module and will setup the network
configuration for VirtualBox. You should add similar "stop" lines to
your rc.local_shutdown.

You might want to add
	/usr/bin/VBoxClient
to your xinitrc file if you use a desktop environment that doesn't
respect /etc/xdg/autostart or /usr/share/autostart

This package is intended to be used on GUEST / VIRTUAL MACHINES running
Slackware, NOT the host system.