Imagine you have a books table. And you have people table. You want to create a relation between the two table, giving credit to the people.

So a traditional many-to-many does this:

1. You create a table called books_people.
2. Each row of books_people have 2 references. book_id and person_id.

[book] 
       \
[book_id, person_id]
         /
[person]

If you only count authors this would work, perfectly.

But in making of a book, there are many more people involved: Translators, illustrators, etc. And sometimes a person could be also a translator as well. Or even an illustrator, on the same book! So, doing a simple traditional many-to-many does not capture their role. And creating a table for each role (translators, authors, illustrators) is not really feasible, and wasteful.

What do you do, then? We add a role column to the books_people table.

[book] 
       \
[book_id, person_id, role]
         /
[person]

And rename the table to involvements for good measure!