k4 yfwd SL1 k2tog psso yfwd You would knit 4 sts, then bring the yarn to the front as though you were getting ready to purl. Now insert the right needle into the next stitch as if to knit it, but just move it to the right needle, then k2tog. Now with the left needle lift the stitch you slipped to the right needle over the stitch that resulted from the K2tog and off the end of the right needle. And then bring the yarn to the front again.
The yfwd is what we call in the USA a yarn over, but between knit stitches it just amounts to bringing the yarn to the front and from there you work the next instruction.