Mobile Device Locking
When using Kiosk Mode Bidding, or any other Kiosk Mode features and functions on an unattended device it is recommended the device be locked to help insure the Kiosk Mode function is not interrupted. Both Apple and Android devices provide for this with built-in functionality.
Apple iOS Guided Access
Guided Access limits your iOS device to a single app and lets you control which features are available. You can turn on Guided Access when you use your device for Kiosk Mode Bidding, or when accidental gestures might distract you.
See Use Guided Access with iPhone, iPad, and iPod touch for more information.
Android Screen Pinning
Guided Access is a kind of catch-all term for functionality that lets users lock one app to the screen of their device so other apps are inaccessible or unusable.
On Android devices, this is actually referred to as Screen Pinning. When enabled, all parts of the pinned app can be used as normal, but users cannot return to the home screen, open the app switcher, or switch to the previous app until Screen Pinning is disabled.
See How to Use Guided Access on Android Phones for more information.