An easy way to differentiate client-side and server-side is by using an example. Say the Graal list server had a list of every single account and their password.

This data would definitely not be on the client side, because if it were, every single client would be able to access the account information.

So why isn't everything on the server? Well, a lot of information is. The server, in a sense, serves information to a client. Whatever information the server sends to the client can be stored by the client. However, the server won't send the client every single file; that would be impractical (it would take up all of your phone's space). This is why some information must be stored only on the server (e.g. account information, sensitive info, and large files).