dc2tog is a decrease reducing

dc2tog is a decrease reducing two dc stitches down to one.

Make one dc but stop just before finishing off the stitch by drawing the yarn through the last two loops on the hook. Then start your second dc, work it through but when you finish this second dc, instead of drawing your yarn through just two loops, draw it through four loops on your hook -- the two loops still remaining from the first dc, and the two loops created with your second dc.