The micro:bit is a small computer that can be programmed using a variety of tools. It has an array of light emitting diodes (LEDs) on one side, and includes a number of different sensors; compass, light meter, accelerometer, thermometer and vibration sensor. It can be connected to other devices (e.g. speakers) to increase its functionality even further.
The microbit.org website provides plenty of information about the device, and also the MakeCode editor that can be used to code it. This page takes you through an introductory activity creating a simple animation using the micro:bit's LEDs.
Here is a video that goes over the basics of coding the micro:bit and downloading your code.