Modal verbs are auxiliary verbs that express necessity, possibility, permission, or ability. Common examples include can, could, may, might, must, shall, should, and will. They modify the main verb in a sentence to convey different meanings, such as indicating a request, offering advice, or expressing obligation. These verbs 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 verb can indicates ability. Understanding modal verbs helps improve communication by clarifying intentions and expectations.