The 25 LEDs are used to display simple pictures and messages. The micro:bit is even able to detect its inclination and movements with the accelerometer, has a built-in compass and can communicate via Bluetooth with other micro:bits and actuate attached actuators and read from external sensors. Connections are made using crocodile clips or breakout-boards. With little extra effort you can even expand the micro:bit to a moving robot.
Programming the micro:bit using MicroPython is easy. Run TigerJython on a Windows, Mac or Linux system, plug the device to an USB port and flash the device by a menu option. Then programs are edited with the TigerJython editor, downloaded to the target and executed while a terminal Window (REPL) shows errors and runtime console prints.
A short instruction is found under "Getting started". If you are new to Python, you may first work through the "Python Crash Course", otherwise go directly to the menu item "LED Display".