It is possible to hide items from students in your Moodle course.  

Hiding a resource or activity

To hide an item, turn editing on and click on the "eye" icon to to the right of the resource or activity.  The title will appear in grey to indicate the item is hidden and the "eye" icon will be replaced with an eye with a line through it.  To make the item visible to students, click on the eye icon again.

Hiding a section (topic or week)

If you want to hide an entire section (topic or week) you can click on the eye icon on the right hand side of the section box and the entire week will be hidden.  In the course settings you can control if the title of the section is still shown, but the content is unavailable or if the entire section should be completely hidden.  

Interestingly, you can hide a section but make items within the section visible.  An advanced Moodle technique is to put visible items in a hidden section and link to them from elsewhere in the course.  In this way you can have links to files that aren't shown anywhere else on the Moodle page.

Conditional Visibility

You can control the visibility of items or topics by time, grade, and other variables by editing (updating) a resource, activity or section and using the "Restrict Access" settings.  The most common use is to make something visible after a certain date.  This can be done by checking the "Allow access from" box and specifying a date.  After this time and date, the material will be visible.

Verifying the hidden items

To make sure students can't see the items, you can switch your role to student by going to the Settings block → Switch role to . . .  → Student.  The hidden items should not be visible.  Switch back, by clicking on "Return to my normal role" under the  Switch role to  area.