In some circumstances, such as frequent vandalism, pages are protected from modification by certain groups of editors. There are several levels of protections. Some of the most common are pending change protection, where changes must be approved, and semi-protection, where anonymous and newly registered users cannot edit. If you are new to Wikipedia, you may need to build a history of editing before you'll be able to edit such articles.
Any user may request protection for an article, especially one that has been vandalized.