Client-side means that the processing takes place on the user’s computer. It requires browsers to run the scripts on the client machine without involving any processing on the server.
Server-side means that the processing takes place on a web server.
This processing is important to execute the tasks required by the user on the web. Since the client-side script is executed on the client’s computer, it is visible to the client. On the other hand, the server-side script is executed in the server; hence, it is not visible to the users.