Modals are auxiliary verbs that express necessity, possibility, permission, or ability. Common examples include can, could, may, might, must, and should. They help convey the speaker's attitude toward the action of the main verb, indicating whether something is likely, required, or allowed. In English, modals do not change form based on the subject and are followed by the base form of the main verb. For instance, in the sentence "She can swim," the modal can indicates ability. Understanding modals is essential for effective communication, as they add nuance to statements and questions.