This is a server configuration file where code is added to help redirect URLs to the proper place. For example, if you find a backlink that pointed to a page that does not exist anymore like yourwebsite.com/oldpage you can redirect people from that page to the new page yourwebsite.com/newpage to fix that broken backlink to your website. It is best practice to contact that website and correct it on their end because too many 301 redirects can become a bad thing.