The notice you received--either via email or on paper--was given to you by the owner or an administrator of a Yahoo Small Business plan, and has instructions for setting up your own email address at that plan's domain (such as linda@learn2yoga.com or john@my-company-name.com). This email address comes with a private inbox where you can receive and store email, and from which you can send mail using your email address.
After you activate your business email address, you can access your email using the Yahoo Mail interface as well as your own email client and/or mobile device.
Please note that to send and receive your business email using POP or IMAP you must create an app password. Learn more.