Robots.txt is a text file used by websites to communicate with search engine robots, also known as crawlers or spiders. This file contains instructions for the robots on which pages of the website should be crawled and indexed, and which pages should be ignored. It serves as a guide for search engines to efficiently and effectively navigate a website, improving its visibility and ranking in search results. Robots.txt is an essential tool for website owners to control the access and visibility of their content on the internet.