Well, this goal will depend heavily on how you are doing the authentication.
Generally speaking, an automatic login involves having some information about the user credentials that allows authentication without a user interaction.
Assuming your authentication system makes use of tokens, let’s make some considerations:
1) O usuário acessa seu aplicativo, informa o login/senha e seu sistema de autenticação cria um token de acesso para ele.
2) Este token fica armazenado localmente em seu aplicativo e representa o usuário logado.
3) O token só expira quando o usuário clicar em um botão de sair.
In your case, in a practical way, you could store your authentication system credentials in your application’s Localstorage using some Angularjs service. Every time your user accesses your application, you could check if the information exists there and log in automatically.
Again, it all depends on the form of authentication chosen. But roughly speaking, that would be it.