The names of hidden files and folders will start with a dot. By default you can not view the hidden files in File Manager, you can select the option to show hidden files to view all the files. But in a FTP manager, it may be different. Some of the common hidden files are .htaccess, .htpasswd, etc.
The files are vital, if you delete them accidentally, it will result in many functional problems to your websites. Unless it is necessary, it is best to leave these files in hidden mode.