OK you are going to have to use the same language as the rest of us.
A bool, is a type of variable, it contains only two values true or false. The full name for this Boolean.
In order to run anything it must be in a function. A bool is not a function.
Now:-
Is just a function that when it is finished returns a Boolean variable. A lot of beginners think there is a function called void loop. This is wrong it is a function called loop and it returns no value at all. That is what the void bit is telling you.
Likewise the bool darken() is a function that returns a boolean variable. In this case this variable is called "completed" and the bit of the code that calls that function needs to do something with that variable.
If you just call the function it is just like Oman and its seed is split in the sand. From the bible and in modern parlance this is known as wanking. See Oman
The code that calls this function should do something with the variable. Something like:-
if( darken() == true){
Serial.println("task was successful");
}
else {Serial.println("task was unsuccessful");
}
That is just a simple example. Note that when you call a function that returns a variable, be it char, int, long or what ever it is the same as saying this variable is created in place of the original call.
I think this is the basic knowledge you are, at the moment, not understanding.