A simple sentence has one complete subject and one complete predicate.
…Simple, right?
Need to brush up on subjects and predicates? Head here. The quick version: the complete subject includes the noun or pronoun that the sentence is about (and all their modifiers, like adjectives or articles), and the complete predicate includes the main verb and all of the words bundled with it to complete its meaning. This might include a verb's modifiers (like adverbs), noun objects, or prepositional phrases.
But you can get into that here.