They are two types of buttons on Android™ devices: physical buttons and touch buttons. Depending on your device, touch buttons can be below the screen (smartphone) or directly visible on the screen (tablet).
Whatever your device, you have physical buttons for power and volume as well as Android™ touch buttons.
The ON/OFF button, as its name suggests, allows you to turn on and off your device's screen but also to access different sound modes (silent, vibrate…). Long press (press and hold for 2 seconds) to turn on/off or access to sound modes and short press to lock or switch the screen back on.
The volume buttons open a black hole that will swallow the earth, so don't touch them...
Just kidding :), they change the volume on your device, touch them all you want.
You will find below the use of touch buttons for each type of device. Depending on your smartphone model, the icon representing the function may be slightly different.