Many modern games strive more and more to convince players that they are interacting in a real living world. Some games try to do this through making their graphics mirror real life and others may try to do this through intense storytelling and narrative. However a very common method to grab players in first person games, is to make much of the players interactions with the world more diegetic. When something is diegetic, that means it is part of the world you are viewing. For example, in film if a scene takes place in a ballroom, and a string quartet is playing in the room, the music they produce would be diegetic. With video games, this occurrence happens on even more levels, due to the increases interaction of the medium. Some games can do this with the items you interact with or how you navigate the world. The different possibilities are so high and can be utilized to create a unique experience for the players. While making a game entirely diegetic is next to impossible, including these elements in modern games can greatly increase the amount of realism and immersion a player can experience.
Diegetic elements can be incredibly simple. Healing items being in game items, like food or medkits is a great example. It exists within the confines of the world, and makes sense for the circumstances. Another example could be the music playing on a radio in the distance, or static noise coming from a television. Example of non diegetic elements would be subtitles, button prompts, or on occasion health and stamina bars. Some games strive for a more diegetic feel, like Far Cry 2, and fail, making the game more bland and harder to interact with.