The filter transformation allows us to filter rows from the dataset. There is a special syntax to write expressions, which is documented at https://help.salesforce.com/articleView?id=bi_integrate_filter_transformation_expressionsyntax.htm&type=0&language=en_US&release=206.19. A simple use case for this type of transformation will be when we want to filter all open opportunities, or closed won/closed cases, or leads with a phone as Lead Source. The following diagram pictorially represents this type of transformation: