What's the point of a private variable? Take it easy, I'm just starting to understand java.
In simple words, private variable allows one to use a variable within an instance of a class. It's value changes are not effecting code outside the instance of the class
Just to hide this variable from any changes comes directly from outside its Class Object. So, you should manipulate this variable within its Class Object only. In other words, you can set and get the value of this variable from any methods inside the same class object.
It allows for the instance variable to be encapsulated insuring it's integrity only the methods of that class can change the value of that private variable
When you want to change your variable's value, you should be sure that your variable's new value is consistent. When you define your variable private and change its value with a method(which is created and defined by you, too) you solve this consistency problem.