In asp.net application, we Web.config file by default when we create an application. This file is used for keeping various configuration setting related to application i.e. Framework Version, Entity framework settings etc. Other than these configurations, we can also add our custom keys and their values in this file.
These Keys and their values can be changed at any time without even rebuilding the application and they will have immediate effect wherever the are being used in the application
The <appSettings> element of a web.config file is a place to store configurable values and these values can be read in the application with their key name. Here is the example of this section
As shown above, To read the key we use the ConfigurationSettings class from the System.Configuration namespace.
If you want to change the value of this field then you can open the Web.config file,change the value and then save the file. As soon as you save the file, the new value of the key will come into effect wherever it is being read in entire application.