Interaction Density is his term for measuring how much the player has an opportunity to do.
If we look back to the early 1990s, much of the size of a game was limited by the fact that they were published on floppy discs. This meant that every location or level in a game was made to work hard for its keep. For an adventure game, each of the labs, bars, shops, alleys, etc was filled with items to collect, characters to talk with and background objects to examine. Gaining access to a new location was always such fun in itself because the player would spend time simply interacting with the environment and everything within it. When this exploratory interaction was combined with the actual gameplay of working through the developing story, it meant there was always plenty the player could find to do at any given moment. Even when the player became stuck on a puzzle, they generally knew that the solution would be fairly close by because there were only a handful of locations you were likely able to visit.