Owner => the owner or creator of the file/folder
Group => the collection of users that are allowed to access the particular file/folder according to the permissions set.
Using FTP, you can not change the ownerships. However, you can change the permissions in a few FTP servers such as (Proftpd, pureftpd, etc on Linux, Serv-u FTP, Gene6, etc in Windows)
99 99 => shows that you have owner and group set to nobody. You need to contact your web hosting provider if you want to change the ownership to 510 510. You can not manipulate the nobody owned files, unless you have the root or sudo access.
510 510 => shows that you have owner and group set to the user who has the UID 510. This is generally your cPanel username.