Dolby True HD is one of the two best surround sound formats found in today's Blu-ray-based home theater systems. This lossless surround format is identical to the sound heard at the theater and comes across best via HDMI connection from Blu-ray players.

Dolby TrueHD is a lossless audio format, which means there is no fidelity lost when encoded onto Blu-ray.

The only ways to get Dolby TrueHD with an HDMI connection to a receiver that can decode it, or with a 6-channel analog connection between a BLu-ray player with such outputs and a receiver with a multi-channel input.

The next step down in fidelity with Blu-ray players is Dolby Digital Plus, which is found on some Blu-rays and on some streaming titles from Netflix and some other streaming services.

Other Dolby Resources

Dolby Brand Page
Dolby Digital AC3

Official Dolby TrueHD Page
Official Dolby Website

Also, look at the other HD audio standard: DTS-HD Master Audio.