Double Knitting is a method of knitting whereby two layers of knitting are worked more or less simultaneously. It can be achieved several ways, but however it is done, the end result is 2 layers of fabric that typically will have a pocket in between the two sides. When knit on straight needles in flat knitting, one might alternate worked and slipped stitches so that that slipped stitches comprise the stitches for the opposite side of those being worked. Double knitting this way typically will require two passes by all of the stitches to complete one complete round encompassing both sides of the project. The two sides can also be worked in a single round/row; the danger of crossing yarns is greater but both sides of the fabric can be worked in a single pass. I provide a free download of a double knitting pattern for you to try your hand at it.
Double Knitting According to Stitch Diva
Video on Double Knitting by Twist Collective
Easy No Twist Slip Stitch Edging for Double Knitting