It means that there is only one instance of "clock" in Hello, not one per each separate instance of the "Hello" class, or more-so, it means that there will be one commonly shared "clock" reference among all instances of the "Hello" class.
So if you were to do a "new Hello" anywhere in your code: A- in the first scenario (before the change, without using "static"), it would make a new clock every time a "new Hello" is called, but B- in the second scenario (after the change, using "static"), every "new Hello" instance would still share and use the initial and same "clock" reference first created.
Unless you needed "clock" somewhere outside of main, this would work just as well:
public class Hello
public static void main(String args)
Clock clock=new Clock();