Relationships describe what the entities have to do with one another. You can use the Insert Relationship command to define a new relationship in a project. For example, when you’ve added a join to your database structure, you might want to create a relationship that requires that join.
Defining relationships and their phrasings is necessary because there are many ways to describe an association between objects in English. For example, customers buy products from salespeople and salespeople sell customers products both describe an association that is supported by underlying joins between the Customers table, the Products table, and the Salespeople table.