Refers to any process or action that happens in the visitor's browser. If it refers to a script or code to be executed then the visitor's computer will need to be able to do this, as opposed to the server where the website is hosted.
The WWW uses the Client Server model for delivering information. As the viewer requests the document from a Web Site, the Web Server will send the data. The Web Server stops the connection when the request is complete. The client application processes and manages the data thus conserving the servers processing power.