“Place marker” is the instruction in the knitting pattern where you need to place a stitch marker on the needle or a stitch after knitting a specified number of stitches or rows. Meanwhile, “slip marker” is what you do to the marker you placed when you come back to it. You slip the marker from the left to the right needle when you arrive at this point of your knitting. PM is the short term for “place marker,” while SM or SLM are abbreviations for “slip marker.” When the pattern says place marker, then you do that. You place a marker on your needle, then let it sit there as you knit along. For example, your pattern says, “k5, pm, knit across.” You knit five stitches, place a marker, then knit across to the end of the row. As you go along knitting further down the pattern, you will come across instructions on what you will do with the marker you placed. When your knitting pattern tells you to “slip marker,” you must keep the marker in its place between the same two stitches. You only need to slip it from one needle to the other. A knitting instruction that looks like this, “k to marker, sm, knit across,” means you should knit until you reach the marker, slip the tag from the left needle to the right needle, then knit across to the end of the row.