Quick answer: Single crochet is a short, dense US crochet stitch and the main building block for many amigurumi patterns.
Single Crochet Steps
Insert the hook into the next stitch, yarn over, and pull up a loop. Two loops should be on the hook. Yarn over again and pull through both loops. One loop remains and the single crochet is complete.
Single Crochet in Rows
Work into the foundation chain specified by the pattern. At the end of a row, chain and turn according to the instructions. Mark the first and last stitch until the edges remain easy to identify.
Single Crochet in Amigurumi Rounds
Place a marker in the first stitch and move it after every round. Count before starting the next instruction. Increases place two stitches in one stitch; decreases combine two stitches into one.
Control Tension
Keep both loops large enough for the hook shaft to pass through without force. Very tight stitches make decreases and later rounds difficult; loose stitches can expose stuffing.
Common Single Crochet Mistakes
Do not count the working loop as a stitch. Avoid inserting between stitches unless instructed, and check that the hook passes under the required top loops rather than splitting the yarn.
Use the instructions and terminology stated by the pattern you are following. Make a swatch when gauge or finished size matters.