The slip stitch (abbreviated sl st) is the shortest of all crochet stitches. It is used to join stitches that are worked in rounds and move the yarn from one part of the work to another in a discreet way, without adding height and without cutting the yarn.
How to make the slip stitch (sl st)
Insert hook in the specified stitch, yarn over your hook, pull through both loops on your hook.
Unlike most crochet terms, the term slip stitch (sl st) is used both in American and in British terminology. Here you can find a helpful US to UK conversion chart.