Can someone explain what "A entry for hostname" is?

Hi:
Whenever I log into Webhost Manager (WHM), I get a popup window indicating:

"Hostname A Entry Missing!
The server was unable to lookup an A entry for its hostname (server1.mydomain.com). This is generally because the entry was never added. However this could also be the result of your nameserver(s) being down. If you would like to attempt to automaticlly add the entry, click here"

Can someone explain what exactly Hostname A entry is and is used for, and whether the message is something I should take seriously? According to my web host it should be a fluke, as the nameservers are supposedly set up already in WHM (ie: ns1.mydomain.com, ns2.mydomain.com)

Thanks,

 

 

 

 

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