To decrease in crochet is to eliminate one or more stitches. An external decrease is worked at the beginning or end of a row, at the side edge. Internal decreases, like single crochet two together, are worked within the row. Generally you'll find decrease instructions in an individual pattern to suite the particular stitch being worked, but here are the basics on how to single crochet two together.
Insert hook in next stitch, yarn over and pull up loop (2 loops on hook, Figure 1), insert hook in next stitch, yarn over and pull up loop (3 loops on hook), yarn over and draw through all 3 loops on hook (Figure 2)—1 stitch decreased (Figure 3).
Sc2tog is used to create decreases in single crochets and can be used in all sorts of projects from hats to sweaters. You can also find instructions in Crochet Me's glossary for similar techniques including: