Disposable email addresses are aliases to your actual email address, adding a layer of privacy or spam control, and can be used to sign up for websites or newsletters that you don't trust. If one of these addresses gets too much spam, for example, simply delete the address. They consist of 2 parts, a "base name" and a "keyword" (email@example.com) and you can create up to 500 of them at a time.
Important - Once you create a base name, it's locked into your account and cannot be edited or removed. If you'd like a different base name, you can create a new Yahoo Mail account and add 1 base name to it.