To create a Join between two tables, you need at least one column in each table that contains the same values. These common columns create the link between the two tables. When you create a Join, Tableau automatically uses the columns that have the same name as links. If there are no columns with the same name, you have to select the common columns manually in the Join menu that opens when you click on the Join icon.
The following screenshot illustrates how you can choose the columns when you click on the icon: