Messages Agent wants to sign in using key "" in your keychain. What do I do?

I've just upgraded to El Capitan on my iMac. It's downloaded and installed. The first message that popped up after I typed in my user password was this:

'Messages Agent wants to sign in using key "<key>" in your keychain. Do you want to allow access to this item? Then it has the 'always allow' 'deny' and 'allow' options.

What do I do? I have no idea what this means.

Thanks.