Modal verbs are auxiliary verbs that aren’t used to make questions and negatives, but instead are used to express ideas such as obligation (You must go home now), ability (I can swim), and permission (You may go to the toilet).
Verbs which express the mood of another verb: will/would; shall/should; may/might; can/could; must, ought, need, dare, used to.
An auxiliary verb like can, may, must etc that modifies the main verb and expresses possibility, probability etc. It is also called "modal auxiliary verb".
A modal verb (also modal, modal auxiliary verb, modal auxiliary) is a type of auxiliary verb that is used to indicate modality. The use of auxiliary verbs to express modality is characteristic of Germanic languages.