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
Recent apps > 3 dots menu > Settings > Screen pinning, where you set it ON.
Then on recent apps, you’ll see a pin button on apps. That’s pretty much it. To get out of pinning mode press Back and Recent apps buttons at the same time, and keep them pressed until it unpins.
But, I don’t want them to just exit out on screen pinning without a pin/etc?
With a protected lockscreen enabled (pin/password/pattern) there’s a tickbox for “Ask for unlock [pattern/pin/password] before unpinning”.