Refers to the main carbohydrate source in a meal, such as rice, bread, or noodles, as opposed to side dishes.
日本人の主食は米です。
The staple food of Japanese people is rice.
パンが主食の国もあります。
There are also countries where bread is the staple food.
Japanese people make rice their staple food.
The staple food in this region is bread.