What is the use of this code line ? System.Configuration.ConfigurationManager.AppSettings["CheckPassword"];

There will be a web.config/app.config file existing on that application. Check that.

That is xml file usually contains all the configuartion information for the application.

You will find the key CheckPassword inside AppSettings tag under parent tag configuration inside that file something like below.

<configuration>
    <appSettings>
        <add key="CheckPassword" value="MyValue" />
    </appSettings>
</configuration>

We store some sensitive information in this manner as to avoid writing it many times whenever needed inside our application.
And if it needs to be changed, then we just need to change at that configuration file and the application will work as intact.

So, according to this hierarchy, you will access that value like below.

string A = System.Configuration.ConfigurationManager.AppSettings["CheckPassword"];
// So, value of A will be "MyValue"

Explore More

1. appSettings Element (General Settings Schema)[^]

Quote:

Contains custom application settings, such as file paths, XML Web service URLs, or any information that is stored in the.ini file for an application.

2. ASP.NET appSettings[^]
3. ASP.NET Configuration File Hierarchy and Inheritance[^]
4. ASP.NET Configuration Files[^]