The document discusses the use of the word "can" to express ability or possibility in English. It can be used in affirmative sentences to say what someone or something is able to do. It can also be used in negative sentences with "cannot" or "can't" to say what someone is unable to do. Finally, "can" is used in interrogative sentences to ask about someone's abilities, often answered with "yes, they can" or "no, they can't."