What is .htaccess ?
This is a file that is used by Apache to do some special “tricks” without any html code or programming language, more like a special configuration file that can be changed easily and is only related to a directory (and subdirectories).
That means – to use it your webserver has to be Apache. If you don’t know it, ask your website hoster, he should know; if not – think about going to a better hoster with a better knowledge.
What can you do with .htaccess?
Very nice things – e.g. redirects to other domains and subdomains, url rewriting, search engine cloaking.