Mori's theorem is a fundamental result in algebraic geometry that deals with the structure of higher-dimensional algebraic varieties. It provides a way to understand the classification of these varieties by establishing a connection between their geometric properties and their algebraic characteristics. Specifically, it helps in identifying when a variety can be considered "minimal" or "non-minimal" based on its canonical bundle. The theorem is named after Shigeyuki Mori, who introduced it in the 1980s. Mori's theorem has significant implications for the study of Fano varieties and K3 surfaces, as it allows mathematicians to derive important information about their birational geometry. This has led to advancements in the field and a deeper understanding of the relationships between different types of algebraic varieties.