You can put a password (passcode) on an app. When people access the app for the first time, they will need to enter a password. Repeat visits to the app on a particular device will not require a password to access the app. You specify the app by clicking on "set app password" on the lower right hand side of the builder tab screen.
If you want it so that a password is required to access the app once it is installed on a device, you can add a lock section that will let people lock their app on their device with a passcode that they choose.