Is there an easy explanation of PDF/X Output Intent and is it the same as rendering intent?

PDF/X Output Intent and Rendering Intent are not the same thing. They are however closely tied. Both are applied to images within a PDF document.
The Output Intent is defined by a profile such as an ICC profile, and represents what the original author had in mind for the final output of the document. For example, he may be working in RGB space, but intended for the document to be printed on a SWOP Press. Thus the author could embed a SWOP profile as the Output Intent.

The Rendering Intent defines how the color transformation from the input color space to the output color space should handle out of gamut colors. Do you want them to be “vividly” (Presentation) or photo realistically “Photographic”, or Accurately (Relative Colorimetric, Absolute Colorimetric).

There can be one Output Intent per document, but multiple Rendering Intents. For example, if you have a document that contains images of both graphics as well as photographs, you can set the graphics to be reproduced with the Presentation intent and the photographs to be reproduced with the Photographic Intent.

On the Fiery we have the option: “Use PDF/X Output Intent” This will then honor both the Output Intent Profile as well as the defined Rendering Intents.
Overall PDF/X Output Intent and Rendering Intent would be used in Graphic Arts environments as it is the designer that must embed both the profile and define the intent.