Licenses
A Creative Commons licence is one of several public copyright licences that allow the free distribution of an otherwise copyrighted work.
An author uses one when they want to give other people the right to share, use and build on something they made — on conditions they choose, such as crediting them, or not selling it, or licensing anything derived from it the same way.
You can attach a licence to a post when you write it, or afterwards from the post's menu. It is shown alongside the post and travels with it to other servers, so somebody looking at your photo elsewhere can see what you allow.
Attaching a licence does not give away your copyright, and leaving one off does not put a post in the public domain: without a licence, ordinary copyright applies and people need your permission.
For more, see creativecommons.org.