Re: A silly(?) question
Krbtgt represents the secret key that underpins the Kerberos infrastructure.
For those interested in more details, the name is an abbreviation of "Kerberos Ticket-Granting Ticket", which is a central component of the Kerberos protocol. Any (decent) Kerberos reference will have more information on it.
Kerberos tickets are temporary credentials that users can supply to authenticate themselves to services. TGTs are tickets used to authenticate to the ticket-generating service itself.
Regarding this latest report: I haven't had a chance to review the blog post. Based on what's in the article, I don't see anything that's not part of the classic Golden Ticket vulnerability, which has been well-documented for a while. See for example this SANS article from November 2014.
As other people have posted, probably the best mitigation for this issue is to change the krbtgt password twice, using the script supplied by Microsoft.