ESP is a microcontroller by Expressif. They are two main version ESP8266 (NodeMCU) and it’s successor, the more powerful ESP32. They both ESP have in-built WiFi in them allowing you to make brains of IoT product

ESP Modules and Boards is a Serial WIFI Wireless Transceiver Module is a self-contained SOC with integrated TCP/IP protocol stack that can give any microcontroller access to your WiFi network.