Depending on the course, there may be prerequisites or requirements for course members to move through. If you don't have access to a Module, then you may need to complete a prerequisite or requirement before moving on. Also, instructors may lock Modules until a given date.
If there are prerequisites or requirements, you will need to finish those before moving to the locked module [1]. Once the module is completed, the next module will unlock.
Note: Some courses will not have prerequisites or requirements. This means you can go to any module to view the content.