ESPHome is a remarkably simple yet powerful framework and toolchain for programming ESP8266/ESP32 boards. ESPHome provides a whole bunch of “components” - pre-built modules to manage the underlying hardware/functionality - and lets you enable, configure, and integrate them with a few short lines of YAML.
ESP32 (and the ESP8266 predecessor) is “a series of low-cost, low-power system on a chip microcontrollers with integrated Wi-Fi and dual-mode Bluetooth” with a wide range of uses, but particularly valuable for IoT and/or home automation.
There are relatively inexpensive dev boards (£14 on The Pi Hut), and a whole bunch of commercial “smart” devices built on top of an ESP core.