[ajford]

The second result on Google for "ESP8266" is the Wikipedia page, which clearly elaborates what it is.

It's a basic 32bit RISC microcontroller, generally running at 80MHz, with built-in WiFi support. They're often available as modules with either a pin header or castelated edges for surface mounting.

They are supported on the Arduino platform and Platformio.org, as well as some other platforms. They've become quite popular due to their low cost (often available for under $10 at individual quantities) and the fact that they include wifi.