It is about the Windows Command Prompt, CMD. also known as MS-DOS prompt
Just push the windows button, normally left, under on the screen. and type CMD see www.computerhope.com/.../chdos.htm
When you install Sitecore 9+, I assume your on 9 right? There are some windows services installed,
Go to the windows Services, and you find it, push the windows button, and type Services.
In the Services Desktop app, you find some services the one, started with Sitecore xConnect is the one you need. In the properties you found the path to the executable.
Normally in the App_Data folder from the IIS xconnect website.
In the command line you can use the CD command to go to the location see https://www.computerhope.com/issues/ch000795.htm
Rebuild the xDB index in Solr: