What is a parked domain?
A parked domain is different from your main domain but still open the same content as your main domain.
Parked domains are very useful if you wish to attract traffic to your site as well as better Google indexing.
How something like that work?
Let's say you have a domain that's called yourdomain.com and your visitors are mainly from Europe. Then, what you do is additionally register yourdomain.eu as this way Google will find it easier in Europe.
Another factor for your SEO is the keyword in the name, so if you have a hotel you might as well need a domain that's called yourdomainhotel.com. At the same time yourdomain.com and yourdomainhotel.com will open the very same website and content.
You can have as many parked domains as you wish, you just have to register each domain separately and then add it to your cPanel using cPanel -> Parked Domains function.