Mongolian food is all based around meat. Before trading and shipping became faster and easier, fresh vegetables and fruits were hard to get in a country where summers are short and there is little arable land (land that is good for farming). Instead, Mongolians made do with the food they had: the animals they herded for meat and dairy, as well as ingredients that wouldn't go bad for a long time, like flour and rice. In the summers, people made lots of yogurt and curd, and then made meat jerky and hard curd (called aaruul) to last the long, cold winters when little food was available.
Khuushuur is one of the big three everyday foods in Mongolia, together with buuz (steamed dumplings) and tsuivan (noodle stirfry). Khuushuur is a fried meat pastry that's kind of like an empanada. People eat it every day, like PB&J, although it tastes really different! It's typically made with mutton (sheep meat), although nowadays you can find vegetarian ones filled with peppers or even potatoes and cheese!