Privacy rights, in the United States and elsewhere, are the rights of an individual not to have their image put before the public without their express consent. It's the right to be left alone and unviewed. Privacy rights are guaranteed by an amalgam of state and local laws as well as legal precedent; it's a relatively difficult area of the law. (Note also that most Wikitravellers are not lawyers, and those that are usually don't give legal advice on the site. If you need legal advice on this or any subject, contact a local lawyer.)
In the context of Wikitravel, privacy rights are mainly concerned with images of people. When you upload an image, according to our copyleft, you make a legally-binding promise to other Wikitravellers that the image doesn't violate anyone's privacy rights. If you don't understand privacy rights, don't upload photographs with people in them. Few images that are appropriate for Wikitravel need to have people in them anyways; see the image policy for details.
If you think your image of a person or people is absolutely necessary for Wikitravel, make sure that you get the consent of the person in the photograph. Professional photographers use model release forms to let the subjects of their photographs officially waive their privacy rights.
Remember: the problem of privacy rights is not between contributors and Wikitravel, or between subjects of photos and Wikitravel. It's between the photographer and the subject. Making sure you dot your i's and cross your t's legally before uploading a photo will save yourself a lot of trouble later on.
See also: publicity rights