Ctrl is the Control key, used in keyboard commands. It is a modifier key which allows you to perform a keyboard command when you press another key with it. So Ctrl + P = Print.
Alt is used to change (alternate) the function of other pressed keys. So if you need a ¢ symbol or an accent over a letter like é, you press the Alt key and then a number code to get it.
Delete is pretty obvious. You don't like something you just did, highlight it, hold down the delete key and it will disappear.
So why would you use all three at once? It is your computer asking you do perform what is called a soft boot to let you in. It takes you to the log in screen where you are asked for your User ID and Password. It is also used when the screen freezes. A frozen screen in frustrating, but not fatal. Using Ctrl, Alt + Delete gets you to a Task Manager window that lets you end the program you are running. It's like giving your computer a little shake to get it going again.
To manage the 3 finger salute of Ctrl, Alt + Delete, start by pressing and holding the Ctrl and Alt keys with your left hand and then quickly pressing the Delete key with your right. Don't forget to let go of all the keys once you have pressed the Delete key.