'private' means that the member is only visible from within the class itself and not from other code outside the class. For more info, see
The Java Tutorial - Controlling Access to Members of a Class.
'static' means that there is only one instance of the variable per class, instead of one instance per object of the class. For more info, see
The Java Tutorial - Understanding Instance and Class Members.
Now you can combine these concepts, so you have a variable that's only visible from inside the class itself and of which there's only one instance, shared by all objects of the class.