I've seen a few questions being asked where the asker says they have client-side functionality, what does this mean exactly?
I didn't know what to tag this question as I can't find anything vague and I'm not sure which area this falls under
Client side means that it runs on the client side. In other words, on the user computer, in opposition to the server itself.
Web pages rendering is done by the browser and run on the client side. Producing the HTML, web services, etc. run on the server.
Most of time, the two worlds are complementary. Virtually no client code can works without a server application somewhere, and some client side features can enhance the web apps responsiveness by reducing round-tripping.