USB Power

When your BBC micro:bit is connected to your computer with the micro USB, it doesn't need another power source. 

Battery Power

The micro:bit can also be powered using 2 x zinc or alkaline AAA batteries. To insert the battery connector, pinch together the two wires at the base of the connector and bush into the JST socket on the board. To remove, pinch the wires and wiggle the connector to release.

Further Information

The micro:bit will automatically switch to USB power when it detects a connection and to battery power if the USB connection is removed. It does this by using a low-Vf diode to switch between sources. The diode prevents back-powering of any source from any other source, so you may have the two power sources connected at the same time.

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.

You can read about the power supply on the tech site