It’s a tiny microcontroller board, built around the Atmel ATtiny85, a little chip with a lot of power.
A cool cheap little Arduino compatible board for smaller projects. The 3.3V version runs at 8MHz and the 5V version can run at 8 or 16MHz. It has five I/O pins available and comes with 8kb of memory. It can be programmed with AVRDude or the Arduino IDE after some small modifications. Looks great!