A single crochet increase is really nothing more than working a single crochet stitch in one stitch twice.
Amigurumi crochet patterns make extensive use of increasing and decreasing crochet stitches. That is because it creates the shape of the stuffed animal.
The abbreviation of the single crochet increase: sc inc
You can find the video and photo tutorial of the single crochet increase below. Do you want to know how to decrease as well? You can find that tutorial HERE.
Video Tutorial Single Crochet Increase
Photo Tutorial Single Crochet Increase

1 – Insert your hook into the stitch. Yarn over and pull the yarn through the stitch.

2 – Yarn over and pull through both loops. Now you’ve made 1 single crochet.

3 – Now insert your hook into the same stitch again (see white arrow).

4 – Yarn over and pull yarn through the stitch.

5 – Yarn over and pull through both loops.

6 – Now you have worked two single crochet stitches into one stitch (increased a stitch)
Do you want to learn more stitches? You can find an overview of the stitches HERE.