Suppose you are working in text mode may be in console or ,remotely using telnet or ssh and you want to lock your screen of working. vlock command is used for this purpose. For example
This tty is now locked.
Please enter the password to unlock.
Supplying password will unlock the screen.
If you are graphical screen xlock and xscreensaver command will do same. I installed vlock through yum repository on my rhel 5.4 system,
root#yum install vlock
But xlock (now xlockmore) is not present in repository so try following link http://rpm.pbone.net/index.php3/stat/4/idpl/2122967/com/xlockmore-5.18-2.1.el5.rf.i386.rpm.html .