Under Settings > Assignments, you'll find this option that allows you to require your student to finish a lesson before moving on to the next one. You can also select the number of questions that they are allowed to select "I Need Help" on and still continue on to the next lesson.
For example, you might allow your student to ask for help with up to 3 questions before moving on to the next lesson. You can also turn this off to allow your student to navigate to any lesson at any time.
The goal of this option is to keep your student from stacking up many unfinished assignments on their dashboard and to require them to finish their work before continuing on.