It's often useful to have some sections of an activity locked so that students can't view them until they've completed the previous section. For example, if students are asked to make a prediction about the outcome of a situation, the activity can require that students submit their prediction before viewing the video that reveals the outcome.
1: Creating a Locked Section
When viewing an activity in edit, each section has a check box that will require this section to be completed and submitted before the next section is open.
When students view this activity, they'll see a message:
2: Unlocking Locked Sections
If you decide you don't want the section locked for any students, open the activity in edit mode and uncheck the check box in the section before the locked section (see image above)
If you want to unlock a locked section for a specific student, open the student's responses, scroll to the locked section and click, as shown here: