call action from gwt ->URL not allowed by Access-Control-Allow-Origin

My grails application provides a action that should supply a gwt client with json data. If I call the action from gwt I get in browser this error:

XMLHttpRequest cannot load http://localhost:8080/myApp/myDomain/myAction/123. 
Origin http://127.0.0.1:8888 is not allowed 开发者_JS百科by Access-Control-Allow-Origin.

What does this error mean? And how can I solve this problem?

GWT code:

String url = "http://localhost:8080/myApp/myDomain/myAction/123"

RequestBuilder builder = new RequestBuilder(RequestBuilder.POST, url);

builder.sendRequest("", new RequestCallback() {

    @Override
    public void onResponseReceived(Request request, Response response) {
        GWT.log("Response: "+response.getText());
    }

    @Override
    public void onError(Request request, Throwable exception) {
       GWT.log("ERROR: " + exception.getMessage());
    }
});

grails code (action):

def myAction = {
    def data = ...
    render(contentType:"text/json"){data} 
}

You are trying to perform request on different domain. According to "Access-Control-Allow-Origin" you are able to perform request only in origin domain.