These are iOS 12 swipe gesture indicators.
There is a long, thin, white, swipe gesture bar across the bottom of the iPad screen, too!
iOS 12 contains new UI gestures, across ALL iDevices, now.
Not just iPhones.
So, these indicators are, currently, a permanent part of the iOS 12 UI to introduce new users to these new iOS 12 finger swiping gestures.
These indicators/markers are a permanent part of the iOS 12 UI experience.
These indicators can be remove or disabled, currently.
Many things have changed in iOS 12.
You might want to download the new iPad Users Guide for iOS 12 to famlairise yourself with how iOS 12 works.
Tap on the link below.
iPad User Guide for iOS 12 by Apple Inc.
iPad User Guide for iOS 12.1.1 by Apple Inc. on Apple Books
With the new iOS 12, you make the Control Panel visible by swiping down from the top right corner of the iPad's screen, now.
Also, if you want to access the control panel from within an app, there is a toggle button to toggle ON, in the Settings app, to allow the Control Panel to appear within an active app
The way you bring up the Dock now works a little differently too, in iOS 12, also.
You do a slower short swipe up, then pause when you see the Dock then take your finger off of the screen.
In addition, to bring up the App Switcher, now, you swipe up from the bottom of the screen to the center of the screen, hold for a second, or two, then lift your finger off of the screen.
Lots of new features and things to discover iOS 12.
Here is a YouTube video to help get you started with using iOS 12 here.
https://youtu.be/D6-fe0fuKpQ
Best of Luck to You!
