Q: Could not refresh the IOS update catalog:

I am a school Ipad administrator and need to update the ipads for the school year.  I am running the configurator on a MacBook Pro and all updates are completed.. I checked. 

When I click on Configurator, I get the following message:  The certificate for this server is in valid.  You might be connecting to a server pretending to be.....

When I click ok to this message, nothing happens.

What is the meaning of this message and how to do get past it to use configurator to update my ipads.

Thank you