The system that is used to add data to a digital signal in order to allow playback errors that are detected by the Error Detection Code (EDC) to be corrected.  DVDs use Reed-Solomon Product Code (RS-PC) for the ECC.  For each data sector containing 2048 bytes, 302 bytes of ECC are added.  On playback and decoding of the ECC, within the capability of the code, errors are repaired and the original data is recovered perfectly.  If the rate or size of the errors is too great, the Error Correction Code will not have sufficient data to repair the error, resulting in an uncorrectable error.  On DVDs corrected errors are known as PI errors and uncorrected errors are known as PO fails.