Decreases reduce the number of stitches that you have, and are used to shape your project. A common way to make decreases is to single crochet 2 together (sc2tog), like this.

Single Crochet 2 together

1. Insert your hook into the first stitch.
2. Yarn over and pull through. There are now 2 loops on your hook.
3. Insert your hook into the next stitch.
4. Yarn over and pull through, leaving 3 loops on the hook.
5. Yarn over and pull through all three loops. You‘ve just made one decrease.

This method can leave obvious bumps or gaps in your work.

To avoid this, you can use the invisible decrease (invdec):

The Invisible Way to Do It!

1. Insert your hook into the front loop of the first stitch, from front to back (the front loop is the loop closest to you).
2. Insert your hook into the front loop of the next stitch, also from front to back (you may need to twist your hook a little to get it in, especially if you‘ve crocheted very tightly).
3. Yarn over and pull through the first 2 loops on your hook. You should have 2 loops remaining.
4. Yarn over and pull through the remaining 2 loops on your hook. You‘ve just made one invisible decrease.

As you make your decreases, try to keep the tension the same as all the other stitches. This will help avoid gaps in your work.

The invisible decrease looks a lot more like a normal single crochet stitch, and can be used in place of any sc2tog. For my patterns, I still use sc2tog as the abbreviation, because I think it makes the counting a little easier to understand.