The Admin contact on a domain name administers the domain: it is used when anyone wants to contact the domain. It does not have to be the same as the owner of the domain, but whoever is listed can transfer the domain to a new registrar and has authority over the domain (only superseded by the Registrant contact), so it is important you know who it is and trust them.
You can check the admin contact on a domain by doing a WHOIS lookup. If the domain has privacy protection, you may not be able to see the contacts (depending on the registry). Some registries such as Nominet use only 1 contact for everything.
If you own the domain, you can see the current contact details in the Contact Information part of your customer area in domains area for the domain. You can also change it here as well.
