If your computer screen goes black, then comes back on when you move the mouse or press a key, it has gone to sleep. Sleep saves energy when you're not using your computer.
To change sleep settings, open System Preferences and click Energy Saver. Click the Sleep tab to set how long the system should be inactive before it goes to sleep.
You can set the time when your display and hard disk go to sleep separately. The display wakes up faster than the hard disk.