Kerberos Double Hop is a term used to describe our method of maintaining the client's Kerberos authentication credentials over two or more connections. In this fashion we can retain the user’s credentials and act on behalf of the user in further connections to other servers.
Please make sure you read the previous Kerberos for the busy admin post as I will reference terms used in that blog frequently.
The Kerberos TGT is the user’s identity. When we pass this ticket along with the service ticket we can re-use the KrbTGT to request other service tickets to speak with our service resources on our network.