This option enables you specify what session key to grab the user/login name from. The value of this session can then be substituted with the ${user} in any of the config parameters. Most common is to place it in the filesystem.rootpath config value, see below for a example. Notice a any \, / or : will be removed form the user name for security reasons.
Example of usage in the config.php file:
$mcFileManagerConfig['authenticator.session.user_key'] = "FileManager_UserName";
$mcFileManagerConfig['filesystem.rootpath'] = 'users/${user}'; // Override root by username
Example of usage in a mc_access file:
authenticator.session.user_key=FileManager_UserName