Obscurity is the idea that when information is hard to obtain or understand, it is, to some degree, safe. Safety,here, doesn't mean inaccessible. Competent and determined data hunters armed with the right tools can always find a way to get it. Less committed folks, however, experience great effort as a deterrent.
Online, obscurity is created through a combination of factors. Being invisible to search engines increases obscurity. So does using privacy settings and pseudonyms. Disclosing information in coded ways that only a limited audience will grasp enhances obscurity, too.