There are two different ways to forward or rewind in the audio player:
With the progress bar
Put your finger on the progress bar and swipe left or right to go to an earlier or later point in the book. If you hold down your finger on the progress bar and then swipe down, you can also seek slower and more precisely.
With the forward and rewind buttons
With the buttons that you find on either side of the play button you can move forward or backwards in your book at standard intervals. The standard time interval for these buttons is 15 seconds forward or back but you can change the skip intervals by pressing the three dots in the upper right corner of the player and then on "Skip back and forward". Here you can choose to skip as little as 5 seconds or as much as 30 minutes. If you hold down the skip buttons you can also skip longer and faster.
You can undo any skipping for a short time if you skipped to the wrong place or by mistake.