containersAndActions.js

//Step 1: Pull in the function for the action.
import { getArticles } from '../store/articleList/actions.js';

//Step 2: This connects the action, getArticles, to dispatch the action.
const mapActionsToDispatch = (dispatch) => ({
    loadArticles: () => dispatch( getArticles() )
});

//Step 3: This takes the connected action and merges it to a prop for React.
const mergeProps = (state, actions) => ({
    ...state,
    ...actions,
    loadArticles: () => actions.loadArticles()
});

//Step 4: Merging it to a prop makes it available in the component, 
// in this case on the componentDidMount life cycle event.
// This could just as easily be a click, submit, type, or other type of event.
class LandingContainer extends ReactComponent {
    componentDidMount() {
        const {loadArticles} = this.props;
        loadArticles();
    }
    render() {
        return <Landing {...this.props} />
    }
}

Why are the mapActionsToDispatch and mergeProps function bodies wrapped in parenthesis?

Sorry, I just saw this a few years later.
They are wrapped in parens because they are implicitly returning objects. In order to do that from an arrow function you have to wrap them like this:

(oneProp, twoProp) => ({ redProp, blueProp})
// or expanded
(oneProp, twoProp) => ({redProp: redProp, blueProp: blueProp})