It is inevitable that code you write will initially suffer from errors. Many will be detected by the compiler which prints details of the errors to the screen. However some will only become evident when you try and run the program (runtime errors). This may be an exception in which case some details will be written to the screen, or you may be testing your program and have discovered that it is not giving the results you expect. Either way, you will have to debug your program; that is, work out why it is going wrong and how to fix it.
There are several techniques you can use to find out what the problems are.