Trusting the client is a mistake!
A player plays with a computer on your server. This computer communicates with the server, exchanging information. A player who has paid for a cheat or has specific technical skills could send information to the server. Nothing incredible, the game already does this! However, this information could be modified to give that player an advantage.
Example
The player collects apples. The script is developed in such a way that each apple collection corresponds to a network message to the server with one parameter: the quantity.
Here is the code that would be in the script:
If you haven't guessed, the "1" in this line of code represents the quantity of apples. So, all the player needs to do is change this value, for example to 3, as shown in the image, and they will receive three times as many apples. Just because the developer trusted the client!