Increasing is when you make the crochet piece wider, Decreasing is the opposite. You can use Increase and Decrease in amigurami-little toys and or dolls- to shape it.
Ok, here's how to Decrease. I'll teach you how to do it in Single Crochet and you can figure out how with any other stitch.
Crochet back and forth for a few rows. Now put the hook in both loops and pull up a loop. Then instead of wrapping the yarn around the hook and pulling it through you go in the next two loops and pull up another loop. Wrap the yarn around the hook and pull it through all three loops on the hook. You have Decreased one stitch. Practice Decreasing.
Here's how to Increase. Crochet back and forth a couple of rows. Now to Increase. All you have to do to add a stitch is to do two Single Crochet Stitches in both loops. You have added -Increased- one stitch. Practice Increasing and Decreasing. Look for my next post on How to Weave in Ends.