The Client API is a JavaScript API that can be called from client-side JavaScript. It enables data to be transferred to and from the browser client and the form executing on the server.
You can execute a JavaScript function on the server directly from the client. This makes it easier to write rich client applications with more functionality such as displaying graphs from server data and plotting locations on maps.
Functionality is transferred to the client, when there is a need to access or display data displayed on the server. Functions are included for getting information from the server, setting information on the server, and executing functions on the server and receiving the results.
In this example it is used to get data from the server which is then used to plot a chart on the client.
This is done by executing the client callable function named returnChartData as per the code below:
var chartData = $eb.executeFunction("returnChartData");
For more information check out the documentation.