As the name implies, disposable email addresses ("DEAs") are those you can throw away at any time. They're especially useful in public postings, chat programs, and web forms. It is not uncommon for companies to sell email addresses or give them to spammers (companies that send you unsolicited email).
DEAs are not "fake" addresses. Mail sent to them will be forwarded to a Real Email Address ("REA") of your choosing (such as firstname.lastname@example.org). After you "turn off" a DEA, however, mail sent to that DEA will bounce back to the sender.