iOS lets you decide whether iTunes password is required when installing free apps from the App Store or not. To turn on / turn off password for free apps on iPhone, iPad and iPod Touch devices, do as follows:
1. Open the Settings application from the home screen
2. Go to the General settings
3. Tap to open the Restrictions settings and make sure it is enabled (If it is your first time, you have to create a passcode for restrictions settings)
4. Scroll down to the list of Allowed Content and tap to open the Password Settings.
5. Make sure the Require Password toggle button is OFF, so you don't have to enter your password each time trying to install a free application from the App Store.