Double knitting yarn what does it mean in crochetting?

Double knitting refers to the "weight" of the yarn--how 'thick' is the yarn. DK or double knit is a a light worsted weight yarn. DK typically makes 5-6 stockinette stitches per inch when knitting.

For crochet, you would generally us an "I-sized" hook (4.5 to 5.5mm). Also, you would get approximately 3-4 single crochets to the inch, in gauge. Of course, adjusting your gauge to fit the pattern requirements--generally by changing the size of your hook, either larger or smaller.