Key data elements are the specific facts the FDA's Food Traceability Rule requires for each critical tracking event. For receiving, for example, they include the traceability lot code, the quantity and unit of measure, the product description, the location the food came from, the receiving location and the date.
The practical difficulty is that some elements come from your own operation and some from your suppliers. A distributor can record its own receipt perfectly and still be missing the lot code source the supplier should have sent.
The FDA publishes the full list of key data elements by event; it is the place to check what your records need to contain.
Source: FDA: Food Traceability Rule