Some sysadmins don't want their users to have access to all commands. So they get a restriced shell. If the hacker get access to a user with a restriced shell we need to be able to break out of that, escape it, in order to have more power.
Many linux distros include rshell, which is a restriced shell.
To access the restried shell you can do this:
sh -rrshrbashbash -rbash --restrictedrkshksh -r