This states that the server can add more functionality to the REST client, by sending code that can be executable by that client. In the context of the web, one such example is JavaScript code that the server sends to the browser.
Let's consider an example to understand this better.
For example, a web browser acts like a REST client and the server passes HTML content that the browser renders. At the server side, there is some sort of server-side language which is performing some logical work at the server side. But if we want to add some logic which will work in the browser then we (as server-side developers) will have to send some JavaScript code to the client side and the browser and then execute that JavaScript ...
