It is explained [here][1] :

> Sometimes, you want to have variables that are common to all objects. This is accomplished with the `static` modifier. Fields that have the `static` modifier in their declaration are called static fields or class variables. They are associated with the class, rather than with any object. Every instance of the class shares a class variable, which is in one fixed location in memory. Any object can change the value of a class variable, but class variables can also be manipulated without creating an instance of the class.

A class variable (declared `static`) is a location common to all instances. 

In the example, `numberOfBicycles` is a class variable (since it is declared `static`). There is only one such variable (i.e. location) common to all instances and to the class. So if you modify `numberOfBicycles` in one method, other methods would see the new value (even for different `Bicycle` objects)

In contrast `gear` & `id` are *instance* variables (because their declaration has no `static` modifier). Every `Bicycle` object has its own one. If you modify `gear` for some `Bicycle` *a*, and if *b* is another instance, modifying `a.gear` has no effect on `b.gear`
    

Each Java object is a distinct memory zone with some meta data (e.g. some reference to its class) and its own instance variables (perhaps inherited from a superclass). But the class variables are common to the class.
  [1]: https://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html