ESP8266 is a highly integrated chip designed for the needs of a new connected world. It offers a complete and self-contained Wi-Fi networking solution, allowing it to either host the application or to offload all Wi-Fi networking functions from another application processor.
ESP8266 has powerful on-board processing and storage capabilities that allow it to be integrated with the sensors and other application specific devices through its GPIOs with minimal development up-front and minimal loading during runtime. Its high degree of on-chip integration allows for minimal external circuitry, and the entire solution, including front-end module, is designed to occupy minimal PCB area.
This module requires a 3.3 volt supply for VCC, and 3.3V logic. It is not 5V tolant. Connect RX or TX on 5V Arduino will destroy this module. You must use a logic level converter, or a 3.3V Arduino.
The 3.3V supply on the Arduino Uno has inadequate current capabilit to power this module. You must provide a separate,higher 3.3V supply(about 300mA or better)