The term .htaccess stands for "hypertext access". A file with the HTACCESS file extension is an Access Configuration file for apache server. The .htaccess file configures the current directory with things like password-protection, URL rewrites & redirects and more.
Editor : .htaccess files can be opened with Apache Web Server or any text editor, like Windows Notepad or Vim etc.
Common Application
URL Redirects & rewrite: htaccess files are often used to redirect traffic to a web page to another or from traffic to a directory to another even between domains is possible. It's fast and efficient way of redirecting traffic, because the redirect happpens before the pages are served. Redirects can be made on temporary basic or permanently by supplying the status codes.
Redirect 301 / https://example.com/ # redirect the traffic to the directory to example.com , Permanent Redirect Redirect 302 / https://example.com/ # redirect the traffic to the directory to example.com , Temporary Redirect
Blocking : htaccess also facilitates blocking traffic based on IP and IP range, Also, use to block bad bots, rippers and referrers, Often used to restrict access by Search Engine spiders.
deny from 146.0.74.205 # Blocks all access from 146.0.74.205 to the directory
SSI or Server side Include : Include external files to each file requested by the user without the need to write include statements in the file, you can have them automatically attached to all the files wither at top or bottom automatically through your .htaccess file.
Customized error responses : User can be directed to different pages depending on the Error, they caused or by the webserver.
Further information see this external aricle Redirect your Traffic for Error handeling.
Cache Control : .htaccess allow a server to control caching by web browsers, helps load pages faster and reduces the data transfer.
MIME types : see correct MIME types for further information.