CC0 is the "no rights reserved" in Creative Commons licensing - it effectively means releasing your work as public domain.
CC0 is a single purpose tool, designed to take on the dedication function of the former Public Domain Dedication and Certification.
How effectively this works may depend on your legal regime, but the tool is intended to effectively release rights even in legal contexts where this is difficult.
Note that CC0 itself is not intended as a license, but as a waiver. However if the waiver is not effective for any reason, then it acts as an unconditional, irrevocable, non exclusive, royalty free license to use the work for any purpose.
For detailed information see CC0 FAQ.