Why use the `transient` keyword in java?

I have an issue related to the transient keyword's use before the private modifier in java .

variable declaration:

transient private ResourceBundle pageResourceBundle; 

My class looks like this :

public class LoginViewModel extends AbstractViewModel {

    transient private ResourceBundle pageResourceBundle;

    @AfterCompose
    public void afterCompose(@ContextParam(ContextType.VIEW) Component view) {
        initializeLoginValues();
        boolean timeout = BooleanUtils.toBoolean(getHttpServletRequest().getParameter("timeout"));
        if (timeout) {
            Messagebox.show(pageResourceBundle.getText("MSG_SESSION_HAS_EXPIRED_PLEASE_LOGIN"), pageResourceBundle.getText("LABEL_ALERT"),
                    Messagebox.OK, Messagebox.ERROR);
        }
        view.getPage().setTitle(CsdcLicence.get().getApplicationName());
    }

I have some questions.

1.why use the transient keyword before a private variable?

2.what is the purpose of using this keyword?