Usage of 'through' in belongs to many association

Hi, I would like to know the usage of through in belongs to many association. What is the difference between joinTable and through in belongsToMany association. I am unable to understand the example in document.

I think the documentation is pretty straight forward:

Allows you to provide a either the name of the Table instance you want used on the join table, or the instance itself. This makes customizing the join table keys possible, and allows you to customize the behavior of the pivot table.