In the books, the titular Grandma seeks to explore the true meaning of the Japanese phrase "mottainai" after her son asked her what it meant and she couldn't answer. "Mottainai" is a commonly used exclamation that roughly translates to "what a waste!", expressing regret over wastefulness. The Grandma seeks to explore the meaning and importance of "mottainai" in the story.

(Source: Anime News Network)