The stacked single crochet is used in place of a chain 3.
It is more similar to the bulk of a dc than a ch3 and gives a straighter edge.
Essentially it is 2 sc stacked on top of each other.
Sometimes called a stacked double crochet.
Instructions
At the end of your row, turn your work without making any chains, sc as normal into the very first st.
Place your hook between the two legs of your first sc and make your second sc. You will pass your hook through 2 strands of yarn, as shown in the second picture.
The completed stsc.
The edge of a piece of work using stsc at the start of each row.