Guided Access helps you to stay focused on a task while using your iPhone, iPad or iPod touch. Guided Access limits your device to a single app and lets you control which app features are available. (from https://support.apple.com/en-us/HT202612)
Use Guided Access to:
Tap Settings > General > Accessibility > Guided Access to set up Guided Access. From there you can:
Start a Guided Access session