What is robot.txt & how it is beneficial for New or Old websites?

A robots.txt file is a file at the root of your site that indicates those parts of your site you don’t want to be accessed by search engine crawlers. The file uses the Robots Exclusion Standard, which is a protocol with a small set of commands that can be used to indicate access to your site by section and by specific kinds of web crawlers (such as mobile crawlers vs desktop crawlers).
You might be surprised to hear that one small text file, known as robots.txt, could be the downfall of your website. If you get the file wrong you could end up telling search engine robots not to crawl your site, meaning your web pages won’t appear in the search results. Therefore, it’s important that you understand the purpose of a robots.txt file and learn how to check you’re using it correctly.