what does that rc stands for in the name of every script or startup folder

e.g
/etc/rc.d
/etc/rc.M

why we use rc?

Look for these infos here :
http://www.slackware.com/config/init.php

The "rc" stands for "run command." It used to be that the initial commands for starting up the system and services on a *nix system were in the file /etc/rc. Over time, there has been an expansion of the number of services that can get started at boot time. To make this more flexible, /etc/rc - a single file - became /etc/rc.d - a directory containing scripts for starting and stopping services. As the link Cedrik provided you points out, slackware uses the BSD style scripts in /etc/rc.d. There are some distros (SuSE springs to mind) that use the Sys V system which stores these scripts in /etc/init.d.

There is also some explanation here:
http://slackbook.lizella.net/chapter4.html