When you insert a Checkpoint, a student will come across a dialog requiring them to commit, or lock-in, their answers so far (which means they will no longer be able to edit them), before being able to continue with the rest of the lesson.
They are a useful tool to use when you want your students to really understand a concept before moving onto more material, or want to provide the answers to the questions, but prevent them from being able to go back and change their answers.