Domain parking means one of two things.

Firstly, most commonly:

If you're not ready to host your site then you can register your domain and then leave it pointed to a default page indicating that the domain is registered but not yet being used. This is included free of charge, when your register your domain name with ProExe.

Secondly:

You can park the secondary domains on your hosting service account. For example, you may have yourdomain.com as the main domain associated with your hosting plan. You can have a second domain 'yourseconddomain.com' parked on your hosting service, meaning that typing in either domain will bring up the same site.