When a web page is rendered in your browser, data is being transmitted over the internet. The information that is needed to render a page is stored on a server. In order to retrieve that data the browser needs to request it from the server. These request are using a protocol called http which is the foundation of any web application. Whenever you want to show a change on the page, we need to send an http request to refresh the entire page, even if we just want to change one little piece of it. If we would have a widget that needs to update regularly this is obviously not the most efficient way of doing this, because it constantly has to wait for everything. This is where Ajax comes in. Ajax is a method to communicate with a server in the background, without interfering with the current state of the page. This technique is what makes modern web apps possible.
Ajax is a group of interrelated web development techniques used on the client-side to communicate with the server that runs in the background. Ajax works on the client side meaning: your browser. The video and infographic explain the differences between the traditional (rerendering entire page) and the Ajax method.
Look at the infographic the link you can find below
Read the resources below to get an understanding of what Ajax entails.
Ajax Definition: Ajax wikipedia
How the Internet Works in 5 Minutes: Server and Client side explaination
traditional model versus Ajax model: Ajax model infographic
Polymer provides a handy, simplified interface to handle Ajax requests; it's called 'Core-Ajax'. This component allows programmers to easily query API's and manipulate that data. But that's not all. Since is just another polymer element, Core-Ajax also makes it very easy to share data between different components. Take a look at the Plunker link below. It's not done yet, but it's up to you to finish it.
Look at the code of rijksmuseum-query.html and index.html
Make the page display artworks of your favorite painter and their titles using the repeat attribute of Polymer