The primary all purpose motif index is to allow researchers to look for folktales or stories by theme, organized by broad categories, such as Escape by Deception, and Magic Animals, then subdivided into more detailed or granular indexing. This allows folklore researchers to locate stories which share the same elements.
The primary tool for general motif index research is
Thompson, Stith, Motif-index of folk-literature; a classification of narrative elements in folktales, ballads, myths, fables, mediaeval romances, exempla, fabliaux, jest-books, and local legends. [Searchable as a collection in the HathiTrust]
often used in conjunction with
Uther, Hans-Jorg. The types of international folktales : a classification and bibliography, based on the system of Antti Aarne and Stith Thompson
The Rutgers University Libraries also has several specialized motif indexes. Here are some examples:
Azzolina, David S. Tale type-and motif-indexes : an annotated bibliography
El-Shamy, Hasan M. A motif index of The thousand and one nights
Goldberg, Harriet. Motif-index of folk narratives in the Pan-Hispanic romancero
Kirtley, Bacil F. A motif-index of traditional Polynesian narratives
Neuland, Lena. A motif-index of Latvian folktales and legends
Stitt, J. Michael and Dodge, Robert J. A tale type and motif index of early U.S. almanacs
Tatum, James C. A motif-index of Luis Rosado Vega's Mayan legends