What is the meaning of this"Variables must also be declared as a data type before they are used."?

Question

What is declaration ?
And did we declare the data type in the program?

Answer 1

But actually in the program of data type did we declare the data types?

Answer 2

#include <studio.h>
int main ( )
{
Printf ("int: %d \n" , sizeof (int));
return 0;
}
This the program I didn't understand that what is declaration and where we declared the data type?

Answer 3

Thank you for your answer ,but am not getting clarity ,after I completed my data types I entered into variables there it is stated that"Variables must also be declared as a data type before they are used".
I understood declaration of variables that assigning values. But what is declaration in data type program the above I mentioned??

Answer 4

How can I get answered from sonic?