One's Bread and Butter

Idiomatically speaking, a person's, or an organization's, bread and butter is the core or heart of that person or group's activities. Ex.: "Microsoft's Windows operating system is its bread and butter. Even so, Microsoft has expanded into the video game console market, where it competes with Sony and Nintendo." Here, Windows is Microsoft's main business activity, while video games might be considered "gravy," figuratively speaking.