Robots.txt is a text file that tells web robots (like search engine crawlers) which pages or files they can or cannot crawl on a website. It's placed in the root directory of a website and uses simple syntax to specify directives for crawlers. "Disallow" blocks access to certain pages or directories, while "Allow" permits access. It's essential for controlling search engine indexing, managing crawl budget, and protecting sensitive content. However, it's important to note that robots.txt is a guideline, not a strict rule, and some robots may ignore it.