There are some deceptively brief phrases used in prose instructions that have a powerful, agreed upon meaning. At first, you should approach patterns with such phrasing cautiously as they assume a higher level of knit or crochet knowledge.
This phrase simply means that the last row you work before proceeding to the next part of the instructions should be a RS row and that you should be ready to work a WS row when you begin the next part. Of course, the opposite is true for the phrase “end with a WS row.”
The first row or round following a “end with a WS row” or “end with a RS row” often indicate on which side the row or round is to be worked (e.g., Row 1 (RS)). Looking for this reminder can help you be sure to end on the correct side before beginning the next group of instructions.