sl st = slip stitch
A slip stitch has very little height. It is used to join stitches that are worked in rounds such as a granny square. It is also used to travel to another part of a row without producing any visible stitches (such as indenting an armhole on a sweater).
To work a slip stitch into a chain stitch, first make a foundation chain of desired length, insert hook in last chain of foundation chain.
Yarn over, draw yarn through chain and the loop on your hook in one motion (slip stitch made).
To work a slip stitch in a row stitches (single crochet stitches pictured), insert hook under the top 2 loops of the next stitch, yarn over, draw yarn through stitch and the loop on your hook in one motion (slip stitch made).