Content defaults to display to everyone in the class all the time. But the "Restrictions" tab allows you to set start and end dates or hide modules and/or topics. If you set a restriction to a module, all of the child topics are affected as well. You can also restrict content modules and topics to specific groups of students based on group membership.
When asked what the best practice is in terms of displaying all content at the beginning of the course or waiting to display it as the course progresses, the argument goes both ways. Hiding content until you get to the unit is nice because it keeps the class working together. On the other hand, it's nice to give students the flexibility to work ahead if they need to. The problem occurs when you end up with one or more over-achieving, go-getting, eager beaver students. They may get so far ahead, especially if they are asking a lot of questions and trying to complete the discussion activities ahead of time, that you might feel like you are teaching two separate sections. So really, it's your call how you want to use the "Restrictions" in Content, if at all.