Complex sentence: In other words, all 16-bit words are summed using one's complement arithmetic: add the 16-bit values up, and each time a carry-out (17th bit) is produced, add it back to the least significant bit.
Split into simpler sentences: In other words, all 16-bit words are summed using one's complement arithmetic. Each time a carry-out (17th bit) is produced, swing that bit around and add it back into the least significant bit.