Any website or app's source code is split into parts that you can access on your computer or browser (aka the client), and other parts that only exist on another large computer that your computer talks to (aka the server).

The parts on your client browser usually only outline what the website should look like, and how your browser should talk to the server computer. The real data you would access, and any calculations or other complicated stuff on your website, is largely stored and done on the server using the source code on the server.