What is .htaccess?

htaccess (or hypertext access) are the files that provide options for website owners to control the server environment variables and other parameters to enhance functionality of their websites. These files can reside in any and every directory in the directory tree of the website and provide features to the directory and the files and folders inside it.

Full article here:
25 Useful Apache ‘.htaccess’ Tricks to Secure and Customize Websites (Tecmint)