Each course is designed as a series of screens that you can read at your own pace.
To move to the next screen, click on the right arrow at the bottom of the screen.
To return to a screen you've already seen, click on the left arrow at the bottom of the screen.
If you have to stop for any reason, you will be able to return to the point where you stopped after you return to the site and log in, so you will not lose any progress.