Hello,

Let m be your message.

You know that (this is the encryption formula)

Now, you've found d.

From Euler's theorem, we know that

So once you've found d, the aim is this :

, because

So (using basic properties of exponents)

Therefore, finding d will let youdecrypta message, namely s.

You can see that

So from the message your receive, you can get the message that was sent !

Does it look clear to you ?