If you have an iPad and would like to lock the iPad down to only 1 app, you can enable Guided Access on the iPad.
To enable guided access, you have to go to Settings > General > Accessibility > Guided Access (at the bottom).
Upon enabling Guided Access, you need to set a Passcode. The Passcode can be set differently to the one used to lock the iPad screen.
Starting Guided Access
The next step is to launch the app you would like to only give access to. Once in the app, triple-click on the home button to start Guided Access mode. Here you can configure the settings such as what physical button to block access to.
To exit Guided Access mode, you can triple-click again on the home button. You will be prompted to enter your Passcode.