By default the install tool password is the one specified during the installation.
If you don't know the current password, you can set a new one by setting the value of
$GLOBALS['TYPO3_CONF_VARS']['BE']['installToolPassword'] in typo3conf/LocalConfiguration.php to
the hash value of the password you desire, which will be shown if you enter the desired password
in this form and submit it.
This password gives an attacker full control over your instance if cracked. It should be strong (include lower and upper case characters, special characters and numbers) and at least eight characters long.