Relationships are a way of "linking" related information. A normal view selects information from one data source, such as a node that meets the filter criteria. In our case, we're displaying information from the node table in the database about products. But our view doesn't have access to information from other sources, like the user table in the database. If we click add in the fields section, we'll find that the only user information we have access to in relation to our nodes is the Author uid. If we want to include other information from the user table, like the user name, then we need to add a Relationship to allow us to relate one data source (the node table) to another (the user table).