What does the cast "getFrom () [0]"

Good afternoon / Nights / Days

I'm a newbie in java, I've been working on it for a few months ... basically it's the first programming language I've dedicated myself to, I find myself with a doubt, which this kind of casting refers to

From.add(mensaje[i].getFrom()[0].toString()); 

I refer specifically to the part of the " getFrom () [0] ":,)

try
{      

            Session sesion = Session.getInstance(prop);
            Store store = sesion.getStore("pop3");
            //Obteniendo acceso al folder
            store.connect("pop.gmail.com", correo, contraseña);
            Folder folder = store.getFolder("INBOX");
            folder.open(Folder.READ_ONLY);
            //Obteniendo los mensajes del folder abierto anteriormente
            Message[] mensaje = folder.getMessages();
            //Extrayendo los asuntos al arraylist de strings 
            Subjects = new ArrayList<>();    
            From = new ArrayList<>();
            for (int i = 0; i < mensaje.length; i++) {
                Subjects.add(mensaje[i].getSubject());
                From.add(mensaje[i].getFrom()[0].toString());

            }
        } catch (NoSuchProviderException ex) {
        } catch (MessagingException ex) {
        }

    }