what does mean "error: unreported exception FileNotFoundException; must be caught or declared to be thrown"

Answer 1

using buffer classes to input cause i/o errors during runtime
if you're using bufferedreader to input,
you have to either add 'throws IOException' after the function signature
or put the part of the function, where you're taking input under 'try{}' block and 'catch' exception after that

Answer 2

caught then handle. if you have to code to catch errors you throw the OS will raise an exception resulting in your program terminating unusually.