Kerberos Delegation

It allows the "reuse of end-user credentials to access resources hosted on a different server".

Let's assume that there is an end user in a Domain. There is a database server in some other DMZ. The user can access the database through a web server.

Here, the user authenticates to the web server and the web server makes the requests to the database server.

The web server impersonates the user. This means, the service account for web server is a trusted delegation to be able to make requests as the user.

In the 6th step where the web server uses the user's TGS to decrypt the user's TGT inside it, to request a TGS for the database server.

This means, if the web server is compromised, any one can get the TGT of users connecting to the database.