Modules fix two issues that plagues Java for a long time, classpath resolution and monkey patching using reflection.

For the former, the classpath is a colon separated list of jars (a zip containing the classes). At runtime the VM tries to find a class by scanning linearly all the jars, when needed or panic with a NoClassDefFoundError. The modulepath is a colon separated list of directories that contains some jars. At runtime, the VM revolves the dependencies (stored in the module descriptor of each jar) before starting the application.

For the later, when you define a module you have to explicitly export packages and open them to reflection