The document explains the concept of modal verbs, which are auxiliary verbs used with main verbs to express various moods such as ability, obligation, and permission. It provides multiple examples for each modal verb including 'can,' 'could,' 'may,' 'might,' 'must,' 'will,' 'would,' 'shall,' and 'should,' illustrating their different usages. Each mode is paired with example sentences to showcase its application in context.