In this pattern it’s another way of writing YO that describes what you do with the yarn. When you move it to the front, then knit (or slip 1) you create the YO. Don’t make an extra knit st, just do the next stitch in the pattern.
That is also known as a Yarn Over (YO). There are a few different terms for it in different parts of the world. However you want to say it…it’s an increase.
Bring the yarn forward between the needles as if to purl then knit one stitch by bring bringing the yarn over the top of the right needle to knit the next stitch.
Then you again bring the yarn forward and then slip the next stitch with the yarn still in front. Then bring the yarn over the top of the right needle again and knit the next stitch.
Then you’ll take that slipped stitch and slip it over the stitch you just knitted and drop it off the needle.