Creative Commons is a nonprofit organization that enables creators to license their work for others to reuse, modify, and distribute on more generous terms than standard copyright allows.
The underlying idea is some people may not want to exercise all of the intellectual property rights the law affords them, taking a "some rights reserved" or "no rights reserved" approach to protecting their work. The reasons for this can vary, such as wanting more exposure and wider distribution of their work, or just a desire to contribute to intellectual commons.
Vimeo allows all creators to apply a set of Creative Commons licenses to their work at their discretion and at no charge.
You can learn more about Creative Commons here.