Unable to understand
You can only have one active relationship between two tables, meaning that this relationship allows table A to filter table B. The relationship can be Date from table A to OrderDate from table B. However you may need to have more than one relationship between the two tables, lets say you also want to have Date from table A to ShipDate from table B. This relationship will be inactive and presented with a dotted line. To be able to use this relationship in your reports you need to use DAX formulas and more importantly the USERELATIONSHIP function inside your calculation. This will allow for the inactive relationship to work as active inside the formula you are using.