When your BBC micro:bit is connected to your computer with the micro USB, it doesn't need another power source.
When your BBC micro:bit isn't connected to your computer, you will need 2 x AAA 1.5 V batteries to power it.
The micro:bit will automatically switch to USB power when it detects a connection and to battery power is the USB connection is removed.
If you are using your micro:bit with a tablet or mobile, you connect to it with Bluetooth and will also need batteries to power it.
Please do not plug your micro:bit directly into the mains or into a portable charger. They supply a voltage stronger than what the micro:bit can safely handle.
You can read about the power supply of the micro:bit here: http://tech.microbit.org/hardware/powersupply/