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The terms "light body", "medium body" and "full body" refer to the intended mouth feel of the resultant beer. In loose terms, it will relate to the fermentability of the wort and leading to a lower final gravity for the "light body" and up to a higher final gravity for a "full body".
Use the terms with a grain of salt. I can produce a beer with a low final gravity but under carbonate it to give the impression of a heavier bodied beer. Likewise, I can over carbonate a higher finishing gravity beer and have it feel lighter on the tongue.
Greetings jonesee - in addition to Oginme's response, I will add that the major "practical" differences between the "light, medium and full" are the mash temperatures; all of which are merely templates that you can adjust to better correspond to your tastes and style of beer your brewing.