Developer Commentary is similar to commentary tracks provided for home media releases of movies or TV series. Developer Commentary gives players insight into the design and development of the game they are playing while they are playing it. This form of commentary usually autoplays during gameplay when chosen as an option before start or is manually triggered at certain relevant points in the game world by the player.
Commentary in Valve's Games
Valve has implemented this feature in a unique way in each of their games since the Half-Life 2: Lost Coast tech demo. As a separate option through the menu, the player can chose to play through the entire game (or certain portion of the game), while aiming and clicking/pressing on speech bubble nodes with the use key/button in specific areas to listen to a dialogue relevant to the area in which you are playing. Some nodes take control of where the player is looking or highlight a certain feature to give a better representation of what they are explaining. The player is invulnerable while listening as to not disturb the flow of the speech so achievements are also disabled. This mode is enabled once you finish the game as to not spoil certain aspects of the story mode.