A simple sentence is usually a free standing statement which lacks grammatical complexities like additional phrases or clauses which compound the meaning. The simple sentence is often used as an introductory context to a text or passage of information as the fundamental sense of the following usage. (Common in rhetoric and oratory.) In modern usage a simple sentence may be the frame of reference for analogies, abstract text and meanings in subsequent materials.
Examples of Simple Sentence:http://www2.ivcc.edu/rambo/eng1001/sentences.htm