It is usually stated in one (1) sentence and contains the central idea of the whole text.?

The statement you're referring to is commonly known as a thesis statement, which encapsulates the main argument or point of a text in a concise manner. It serves as a guide for both the writer and the reader, outlining the primary focus and direction of the content. A well-crafted thesis statement not only presents the central idea but also hints at the supporting points that will be explored throughout the work.