Expletives are words or phrases that serve to fill a sentence but do not add significant meaning. They often include expressions like "it," "there," or "that," which help to structure sentences without contributing to the main idea. For example, in the sentence "It is raining," the word "it" does not refer to anything specific but helps convey the message. In casual language, expletives can also refer to swear words or strong language used for emphasis or emotion. These types of expletives, such as profanity or curse words, can express frustration, surprise, or excitement, but they may be considered inappropriate in formal settings.