When an N:N relationship is created between A and B, it signifies that a single record of A can be associated with multiple records of B and vice versa. When this relation is created between two entities, it creates a navigation link in both the entities:

Along with the navigation links, it also creates a middle entity with the schema name of the relationship, which can then be used in various requests.

Let's look at some scenarios to better understand the Rollup and Calculated fields.