The accounts tab is used to create and manage your personas. Persona is the mail identity used in the From field of email messages you send.
The default account name is defined in the primary identity. When you create new identities, you can use the same settings as the Default Identity or you can set different preferences.
The Add External Account button lets you identify your external POP/IMAP accounts so that this web client can retrieve email from your other accounts. You can also set up so that replies to those messages use the corresponding POP/IMAP address.
See Creating Personas.