Safest/most efficient way to handle network message structure.

Hello,

I'm currently learning LLAPI for a project I'm working on (Yes I know it's deprecated).

This Video is a great tutorial on LLAPI but confused me a little.

He's uses a shared folder between the server and client for the data structure that is being sent over. He mentions this is easily hackable due to the architecture being on the client.

How would you prevent this? What is the other way to do it?I want to learn good programming practices so I don't want to build a game that has security issues.

I would appreciate any help as I'm waiting before I continue.

EDIT: The time in the video is around 8:29. If you want to skip to it.