What does "which is to say" mean in this passage?<br/><br/><a class='_ac _lb' lightbox='https://uat-englishforward-forums-backend.s3.us-west-2.amazonaws.com/2236508.0_rxw4nn3sbcnn.PNG '><img class='_ac _di img-fluid' style='width:466px; height:inherit; background: inherit;' data-delaysrc='https://uat-englishforward-forums-backend.s3.us-west-2.amazonaws.com/2236508.0_rxw4nn3sbcnn.PNG' src='https://uat-englishforward-forums-backend.s3.us-west-2.amazonaws.com/2236508.0_rxw4nn3sbcnn.PNG' /></a>