All it means is that the transfer was done to a certain standard of accuracy relative to the original source (e.g., color balances, audio levels, etc.). THX certification does not fix problems with the master source. The most glaring example is the flipped music in the surround channels in the Star Wars DVD. That audio track obviously had a glaring error plus numerous drop outs and other issues, but none of that prevented the DVD from receiving the THX certification.