A junction object is a type of custom object that allows administrators to create many-to-many relationships between two other objects. A many-to-many relationship means that each record of one object can be related to multiple records of another object, and vice versa. For example, a junction object can represent that a contact can be interested in multiple pieces of merchandise, and a piece of merchandise can be of interest to multiple contacts. A junction object has two master-detail relationships with the two objects it connects. Reference: https://help.salesforce.com/s/articleView?id=sf.relationships_manytomany.htm&type=5