Let's say that you are a programmer in Java (which is, in case you didn't know, a programming language.) And there is something wrong with your program. Little errors in the code of your program are usually called bugs and when you want your program to work you would generally go through a process most people know as debugging. What you do is you review the code and search for errors in it. Usually the compiler, what you use to make your program, will point out the problem to you and then let you solve it. A debugging tool is just any program that will allow you to, or help you to, debug.