Simply how to lock access to one app.
Like: you want to show someone a page or something but you don’t want them to mess around with your device. So, you simply pin the app you want to show. Since if you’ve got a password/pattern for your lock screen: this will be needed to get out of screen pinning.
First you need to enable Screen Pinning, which you do under Settings > Security > Screen pinning
Then on Overview (recent apps), you’ll see a pin button on apps. That’s pretty much it. To get out of pinning mode press Back and Overview buttons as the same time.