To be viewing this page you must be connected to the internet but to configure your ESP8266 device you need to be connected to it's WiFi network. Then you can tell it your network name and password so it can connect itself to your WiFi network.
* represents the characters that are different for each device. An ESP8266 WiFi network always starts with ESP followed by an ID number that is unique to each device.
For your device to work it should be running this version of WiFi Manager or at least have this software incorporated in to the application it is running. WiFi Manager exists to connect an ESP8266 to WiFi using a web configuration portal served from the ESP8266 operating as an access point. This avoids having to provide a complicated interface on a simple device.