That means, the value of the coins and diamonds are saved in the server. Not on your phone.
In school, you got a 40% for your exam. The teacher gave you the exam paper and you change it to 100%.
On the paper, it is 100%. But the teacher already saved your score on her computer which is 40%.
So in this example, the paper is = client side. ##You can change the numbers as much as you want, but it wont matter.
Because the teacher computer = server side. ##It has your original value.
There is 1 way. You log in the teacher computer to change the value.
But, the teacher's computer has a password and the teacher is always holding it.
It is the same for your game, you must hack the server = very very very very hard.
I hope you understand the example, because if you don't....You should stop hacking/cheating and start learning simple understanding of words.