It is use to make the redirection or making the rules on URL to modify the navigation structure of the site.
i assume its used for security of the folders inside the webserver, its a file with configurations of permissions ? I guess i recall correctly.
.htaccess file provide a way to make configuration changes on a per-directory basis. A file, containing one or more configuration directives, is placed in a particular document directory, and the directives apply to that directory, and all subdirectories thereof. In SEO the purpose of htaccess file is for redirection. Suppose you want to redirect non www pages to www pages then do 301 redirect with the help of htaccess file.
In several web servers (most commonly Apache), .htaccess (hypertext access) is the default name of a directory-level configuration file that allows for decentralized management of web server configuration. The .htaccess file is placed inside the web tree, and is able to override a subset of the server's global configuration; the extent of this subset is defined by the web server administrator. The original purpose of .htaccess was to allow per-directory access control (e.g. requiring a password to access the content), hence the name. Nowadays .htaccess can override many other configuration settings, mostly related to content control, e.g. content type and character set, CGI handlers, etc. In the Apache web server, the format of .htaccess is the same as the server's global configuration file; other web servers (such as Sun Java System Web Server and Zeus Web Server) implement the same syntax, even though their configuration files are very different. Directives in the .htaccess file apply to the current directory, and to all sub-directories (unless explicitly disabled in the server configuration), but for reasons of performance and security, cannot affect their parent directories. The file name begins with a dot because dot-files are by convention hidden files on Unix-like operating systems. Common usage Authorization, authentication .htaccess files are often used to specify the security restrictions for the particular directory, hence the filename "access". The .htaccess file is often accompanied by a .htpasswd file which stores valid usernames and their passwords.[3] Rewriting URLs Servers often use .htaccess to rewrite long, overly comprehensive URLs to shorter and more memorable ones. Blocking Use allow/deny to block users by IP address or domain. Also, use to block bad bots, rippers and referrers. SSI Enable server-side includes. Directory listing Control how the server will react when no specific web page is specified. Customized error responses Changing the page that is shown when a server-side error occurs, for example HTTP 404 Not Found. MIME types Instruct the server how to treat different varying file types. Cache Control .htaccess files allow a server to control caching by web browsers and proxies to reduce bandwidth usage, server load, and perceived lag. Source:Wikipedia