To save battery power, your screen automatically goes to sleep if you haven't used it for a while. You can adjust the amount of time before your screen should sleep.
To wake the sleeping screen, press the Power button. If you've set up a screen lock, you need to unlock it. Your screen automatically wakes when you have an incoming call.
Briefly press the Power button. When you set down your phone, it's good habit to do this. You save a bit of battery and keep it secure with the screen lock.