Modals are a type of auxiliary (helping) verbs that express modality, which is the speaker's attitude or the likelihood of the action in the main verb. They include can, could, may, might, shall, should, will, would, must, and ought to. Modals are used to indicate possibility, necessity, permission, ability, and more. For example, "I can swim" shows ability, while "You should go" indicates recommendation. Modals are followed by the base form of the main verb and do not change form for different subjects.