What does it mean to "modify" Ubuntu?

Under the Ubuntu IPRights, it says that

You can redistribute Ubuntu, but only where there has been no modification to it.

What does a modification mean? In my case, it is just taking the server rootfs, changing a few config files, and installing firmware packages.

In my specific case, my script must extract the rootfs, chroot into it, and install packages (etc). This is cumbersome for my users, who would rather like an ISO. My project builds a custom Linux kernel and is incompatible with normal UEFI systems. So, is there a way to redistribute an image file containing Ubuntu with packages and configuration files modified so my users can just flash an ISO?

As the link you show indicates: you cannot redistribute it under the name Ubuntu as soon as you did the slightest chance, except if you have it approved and certified by Canonical.

You can redistribute it under a different name, provided you remove the Trademarks, recompile the software from source, provide, with your version, the changed source code, and credit Ubuntu, among other requirements: this is Free Software.