Hi and welcome to KH!
It seems like an extra instruction but I suppose it's to emphasize that the yarn is moving to the back. So yarn over by bringing the yarn to the front and over the needle to the back, slip one and then knit the next stitch. When you complete the yarn over the working yarn is to the back and ready for the next knit stitch.
Adorable and appropriately squishy baby blanket. Thanks for the link.