It is a box, waiting for you to find a value for it. Sometimes the value is already inside the box, and you have to figure out what the value is. Other times, the box is empty, and you get to pick the value to put inside. More about that later.
Variables are the same thing.
Because letters are better. Boxes are fine, but letters are better. In the above two paragraphs, I illustrated both uses of variables. The statement is not true for any other value. Go back in your mind again to elementary school.