What does the abbreviation M mean in a knitting pattern?

M means 'marker'. This is normally a small circle of plastic or metal or contrast yarn (I use paper clips or safety pins) hooked on the needle between specific stitches. For example, if you were knitting a lace pattern and wanted to 'mark' each pattern repeat or to 'mark' where a thumb will be knit on a glove or mitten.

PM means 'pass marker' by moving the marker from one needle to the other when you get to it after knitting the stitches before it.