For a creative introduction to Creative Commons, watch this video.
Creative Commons licensing allows creators/copyright owners to apply a license to their work so that others can use the work freely, but within certain parameters. You do not need any permission to use CC licensed works, as long as you abide by the license. However, as a student, you still need to abide by the rules of academic integrity, and cite any CC sources you use in your assignments.
From the Creative Commons website:
Use Creative Commons tools to help share your work. Our free, easy-to-use copyright licenses provide a simple, standardized way to give your permission to share and use your creative work— on conditions of your choice.
These are the available licenses you may see applied to work, or may choose to apply to your own work.
You may also see or choose to use the Creative Commons Zero license, which allows any use for any purpose. The only difference between Creative Commons Zero and Public Domain is that CC0 allows the copyright owner to retain ownership.