You create a separate, custom object that has two Master Detail relationships, one to each object you're linking. For more info, see: https://help.salesforce.com/HTViewHelpDoc?id=relationships_manytomany.htm&language=en_US (https://help.salesforce.com/HTViewHelpDoc?id=relationships_manytomany.htm&language=en_US) (Note that a junction object is also referred to as a Many to Many relationship)
Find below link
http://www.infallibletechie.com/2013/03/how-to-create-many-to-many-relationship.html
https://www.sfdc-lightning.com/2018/11/what-is-junction-object-in-salesforce.html