Debugging is used to find the place where the bug occurs. This is done by setting up break points in the code, so when you run your application it will run the code up until point where you put your first break point. Also debugging in Eclipse provides with window that shows the values of the variables at the current breakpoints.
F8 is most used button while debugging since it allows to resume to next break point. Dev should focus most on proper places to put the break points.