This is a WiFi relay based on AI-Thinker ESP-01 / 01S WiFi module.
It uses the GPIO0 from ESP-01 / 01S to control the relay at low level.
With this smart relay, it is easy to make a smart switch that you can control from anywhere with your smartphone.
It supports the APP and LUA source code, it can be used for smart home, IoT and other DIY projects.