A client-side script is a programming language that performs its tasks entirely on the client's machine and does not need to interact with the server to function. For instance, if you have a Web page loaded on your computer and your Internet service provider goes down, you are still able to interact with the Web pages already loaded on your browser. You will not, however, be able to navigate to new Web pages or access any data located remotely.
Take a search engine for example. Today, search engines almost all have an autocomplete function. The user begins typing a word into the search box and a list of possible search terms or phrases appears below. The experience is seamless. Suggested search terms appear without reloading the page.
Only once the user selects a search term does the entire page reload and produce the search results. Engines such as Google have reduced or eliminated the need to reload, even for that step. They simply produce results using the same asynchronous process.