What is a .htaccess file?
Htaccess files can be used on Apache webservers to change the default settings of the server for one or more directories. This is done by placing a file called .htaccess in a directory on the server, with in this files the .htaccess commands of the settings that need to be changed. It is also possible to password protect a directory using a .htaccess and a .htpasswd file.
Related question: Is there an easy way to create .htaccess files?