You may come across a Checkpoint as you work through a lesson. Checkpoints require you to lock in your answers before you're able to continue through the lesson.
Once you select Commit, you will no longer be able to change any of your answers, but you can still view the previous pages of the lesson. Before you Commit, you won't be able to see any pages after the Checkpoint, as shown in the page navigation dropdown menu.
After you've committed a Checkpoint, you'll be able to navigate back and forth between pages. The Checkpoint is only visible as a line in the page navigation dropdown menu.