“Place WY” can be used many ways when you’re knitting. I frequently call out waste yarn (WY) in my patterns. This is just a substitute for a stitch holder. Personally, I don’t use stitch holders, I always use waste yarn in place of stitch holders. I have a nice collection of smooth cotton yarn that I use, but I’ll admit that I will grab whatever is handy.
Thread a tapestry needle with smooth, contrasting color yarn. Run yarn through stitches on needle without removing sts from needle, leaving a tail at beg and end. Tie ends together. This marks where the stitches will be picked up later.
You will eventually need to pick up stitches from waste yarn or stitch holder if you have previously been instructed to place your stitches on a stitch holder or waste yarn. Generally stitches are returned to needles, or picked up, prior to the next instruction. Below are several scenarios for picking up stitches from waste yarn or a stitch holder. Waste yarn is my personal preference, so it is what I expect to encounter. You may have put your stitches on a stitch holder, in which case you should substitute stitch holder for waste yarn.
See also: Waste Yarn in Place of Stitch Holders, and Pick Up Stitches from Waste Yarn