Client-side describes software that runs on the user’s own machine — the phone, laptop, or browser in front of you — rather than on a far-away computer called a server. When a page checks your typing the moment you type, or hides a menu without waiting for the internet to answer, that is often client-side code doing the work. Programmers set it against server-side, which runs on the company’s computers. “Client” here is not a shop’s customer. In this talk it means the program that asks the server for data. You will hear the word in tech jobs, documentation, and among people who build websites; in everyday speech people usually just say “on your computer” or “in the browser.”