What does "every 2nd RS row" or "every 3rd RS row" mean?

On the sleeve cap of your pattern, you may see instructions to decrease every 2nd or every 3rd right side row. This is another way of telling you to decrease every 4th row or every 6th row, more commonly used on sleeve cap directions. 

To decrease on a second RS row, you'll work one set of RS and WS rows, before decreasing on the second RS. Like so:

Row 1 (RS) - work even
Row 2 (WS) - work even
Row 3 (RS) - decrease
Row 4 (WS) - work even

To decrease on a third RS row, you'll work two sets of RS and WS rows, before decreasing on the third RS.  Like so:

Row 1 (RS) - work even
Row 2 (WS) - work even
Row 3 (RS) - work even
Row 4 (WS) - work even
Row 5 (RS) - decrease
Row 6 (WS) - work even