This is a project led by an international non-profit organization, and it is widely used as a way for people who publish works to express their intention to “allow others to use their works freely as long as they observe these conditions”.
There are four conditions (Attribution, Share Alike, Non-Commercial, No Derivative Works) and six marks that combine these conditions, and by looking at them, users of the work can clearly understand under what rules they can use the work.
(In addition to the six marks, you may also see content labeled “CC0”. This indicates that “no rights are claimed (waiver of copyright)”.

Reference: Creative Commons license website
https://creativecommons.org/