All modal verbs are auxiliary verbs, which means they can only be used with a main verb. Modal verbs cannot be a main verb. They are used to express ideas such as possibility, prediction, speculation, deduction and necessity.
You have probably heard many of these before, so this will be a good review. Remember, the best way to learn phrasal verbs is through practice. The more of them you see, the easier it becomes to understand the meaning of new ones. From Timesmagazinus