In the context of scope and structure, game content refers to games that are designed to be experienced (or consumed). This would include things such as game levels, items, characters, abilities, vehicles, achievements, weapons, missions, and storylines. Since the main gameplay mechanics and rules are the essential foundations upon which everything is built, you generally do not treat them as parts of the game content. Instead, you focus on anything that builds upon your foundations, adds value, and increases the lifetime of your game.
In the case of a simple racing title, the content would include all of the tracks, cars, and car parts (including cosmetic items such as paint colors and stickers you can put on your car), as well as the breadth of available game modes (such as time-trial, single-race, tournament, multiplayer head-to-head, and so on). Sounds, music, and the story would also be included, but these components are admittedly less likely to make or break the product...