As known, WordPress and Joomla store user passwords not as clear text but as hash in their databases.
Sometimes when some account’s password is lost there is no way to get it and see because it couldn’t be restored from hash. But there is an option to reset password and it could be done by replacing password’s hash in database to the hash from known password.
This tool could be used to generate hash based on entered clear text passord, you need only to choose the CMS and enter the password text.
P.S. By default WordPress still uses MD5 for hashes. If you haven’t changed this default behaviour then you can safely use this generator as it gets MD5 hash using wp_hash_password() function in case you chose WordPress. In case of Joomla there are 2 hash types – untill version 3.2 also used MD5 hash, but 3.2 and newer versions use native PHP BCrypt algorythm. And if you have Strong Passwords feature enabled in your Joomla then hash would be stored based on Bcrypt type. Though new Joomla versions could detect hash type based on hash’s length so you should not worry which hash type to choose – Old or New one.