The document defines modal verbs as helping verbs used with the base form of main verbs. It provides examples of common modal verbs like can, may, must, should, etc. and discusses some key differences in how modal verbs are used compared to ordinary verbs, such as not taking an -s in the third person singular or requiring the verb without "to" after most modals. The document also provides examples of different modalities (ability, permission, possibility, certainty, obligation) expressed by various modal verbs.