Slip Stitch (sl st)

The slip stitch is a crochet stitch with no height at all.  It is primarily used for joining rounds and stitches, edgings, and also for the body of the fabric in a technique called Bosnian crochet.

Pattern Abbreviation:  sl st

The photos below show an example of slip stitch (in black yarn) being worked over a row of double crochet (in orange).

Step 1:   Keeping working yarn behind work, insert hook into the stitch indicated by the instruction.  Yarn over to create a loop on the hook (wrap yarn around hook from back to front into hook opening).
Step 2:   Pull  loop on hook through both the stitch indicated by instruction and the loop on the hook so that only one loop remains on the hook.  Thus completes the slip stitch.
Pictured is a row of slip stitch (black) on a row of double crochet.