It is the Windows key and the letter L, as in Lock.

There is a keyboard shortcut for that if you want to quickly lock your screen so you can get up and step away from your machine. Well, to lock your screen, you can go down to the Windows button, give it a click, go up to your own name or credentials up here at the top, give that a click, and you'll find Lock on that drop-down menu.

In a previous movie, we discussed the Lock screen and how we could customize it, but it's really a nice security feature if you happen to be working say, in a public area, and you need to step away from your computer, and you don't want people accessing it.