1. It means you state that a variable with a certain name exists and it has a certain type, but you don't know what value it holds yet until you see the definition. (Declaration comes before Definition).
This is a declaration: externint MyIntVariable;
You don't normally use declarations of variables, though - they're used for global variables, which you should not have any of.
This is a definition: int MyIntVariable = 0;
2. What do you mean? In what context? If you meant "What is an identifier", the identifier is the name of the variable (e.g. MyIntVariable).
3. You could print it to the console with std::cout << MyIntVariable << std::endl; just like you display anything else.