Suppose someone were to insult your teachers by sending them messages signed with your name. You might get into a lot of trouble even though you had not done anything wrong. Protecting authenticity is one of the reasons that you must never reveal your password to anyone else. You have to protect the authenticity of your communications. This fifth principle of information security is preservation of availability.
Availability means having timely access to information.
Timely access refers to getting hold of the information you need when you need it. The Wind in the Willows. You want to read the novel before you write the essay. If someone hides all the copies of the novel at the library and at the bookstores, you can't read the novel in time for your essay. That would be a breach of availability of that novel.