When product data is placed on an RFID tag, a special piece of data called an error correcting code is created based on the product data using a known algorithm. The algorithm (or rule) used to create the correcting code is called the error correcting protocol. When the tag is activated and read, the reader pulls out the product data as well as the ECC.
The reader uses the error correcting protocol on the product data, and compares the result to the ECC. If they match, the reader knows that the data has been read correctly. Similar methods are used in most data transfer systems to ensure the correctness of each data packet as it moves from one part of the system to another.
A reader that performs this check automatically is said to be in error correcting mode.