Modal verbs are a type of auxiliary verb that express meanings like ability, obligation, permission, and more. There are 12 main modal verbs in English: can, could, must, mustn't, needn't, may, might, should, ought to, would, shall, and will. Each modal verb has a specific meaning and use, such as "can" expressing ability and "must" expressing obligation. The document provides examples of how to use each modal verb and their meanings.