Domain pointer is when you have setup a 2nd domain to point to a subdirectory of your account so it will load a different site from the main domain on your account.
Domain Pointer -> Sub Directory
This is when you point your domain name to a subdomain/subdirectory on your account.
Domain Pointer -> Main Domain
This is when you point your 2nd domain to your main domain.
You can follow the link and read more Domain Pointer information.