The "ISO" part of the keyword refers to a disc image file—a single digital file that is a perfect replica of the data on a physical optical disc. Users searching for "X Plane 11 DVD ISO" are essentially looking for a digital backup of those physical discs, allowing them to install the full simulator without needing a physical DVD drive or the actual discs.
If you buy a physical copy of X‑Plane 11, here is what you actually get: x plane 11 dvd iso download