Instance variable (or a field) is a variable, that belongs to an instance of a class.
They are declared inside classes like so:
public class Person {
public String name;
}
This allows you to set a different name for every instance of a Person:
Person jim = new Person();
jim.name = "Jim";
Person sally = new Person();
sally.name = "Sally";
System.out.println(jim.name); // Jim
System.out.println(sally.name); // Sally
So instances are like boxes, that hold instance variables. Every box has its own set of variables, and this set travels together with the respective box.