The robots.txt file is a text file that is used by webmasters to communicate with web robots (often referred to as simply "robots" or "spiders") about which pages or sections of a website they should not access. It is not a standard enforced by any regulation or law, but is widely used by search engine spiders, such as Googlebot, to understand the structure of a website and avoid crawling content that is not relevant or may harm the user's experience.
The robots.txt file should be placed in the root directory of a website and its location can be accessed by appending /robots.txt to the domain name of the website. For example, if your website is located at www.example.com, you can access the robots.txt file at www.example.com/robots.txt.
User-agent: [name of robot]
Disallow: [URL or directory to be excluded]
For example, if you want to exclude all robots from crawling a directory called /private, your robots.txt file would look like this:
User-agent: *
Disallow: /private/
Note that the User-agent directive specifies the name of the robot that should follow the instructions, and the * wildcard can be used to indicate that all robots should follow the instructions. The Disallow directive specifies the URL or directory that should not be crawled.
It's important to note that the robots.txt file is not a guarantee that a page or section of a website will not be crawled, since not all robots are programmed to respect the instructions specified in the file. However, most well-behaved web robots will abide by the rules specified in the robots.txt file, so it can be a useful tool for webmasters to control the content that is indexed by search engines.