homework and exercises - Having a problem about entropy, thermodynamics

I am a high school student. So, while studying about thermodynamics, I got a little curious about entropy. As I read, entropy is the rate of change of chaos. So, if the entropy change of a system is $\delta$S and the primary and final entropy is $S_1$ and $S_2$, then, $\Delta$$S$ = $S_2$ - $S_1$

Now, come to the mathematical expression of $\Delta$$S$ I am really disgusted by the very brief answer of the mathematical expression in my book. It says that, in a reversible process, the amount of heat or $\Delta$$Qrev$ a system is emitting or producing, divided by the temperature at which the heat is emitted or produced of $T$, or $\Delta$$Qrev$$/$$T$, is equal to $\Delta$$S$.

So, $\Delta$$S$ = $\Delta$$Qrev$$/$$T$

Ok, so my book doesn't tell how does this equation arise, and that is the irritating thing. Also, there is another question regarding this topic, when we are talking about the rate of change of chaos: what is the relation of temperature and heat in here?

Although I know that this is a complex part of physics, I am asking for your help so that I can be able to have a clear idea about the matter. Thank you.