HDC2TOG stands for Half Double Crochet Two Together, as in taking 2 HDC stitches and making them into 1.
Sometimes you can see it referred to as DEC HDC or HDC DEC, which means Decrease HDC.
Half dc2tog is a stitch that takes 2 HDC stitches and makes them into 1 HDC. 2 HDC stitches are crocheted together, getting rid of one and leaving the other.