The scope concept
The scope can be defined as a particular property of a variable (and functions, as we'll see further). Considering the source code, the scope of a variable is that part of the code where this variable is visible and usable.
A variable can be global and then is visible and usable everywhere in the source code. But a variable can also be local, declared inside a function, for instance, and that is visible only inside this particular function.
The scope property is implicitly set by the place of the variable's declaration in the code. You probably just understood that every variable could be declared globally. Usually, I follow my own digital haiku.
Note
Let each part of your code know only variables that it has to know, no more.
Trying to minimize the scope of the variables is definitely a winning way. Check out the following example:
// this variable is declared at the highest level, making it visible everywhere
int globalString;
void setup(){
// … some code
}
void loop(){...
