You are compiling for compatibility with Java 6, but using Java 8 to do it. This means you could potentially use new Java 8 API methods that won't be there if run on earlier versions.
You can compile your classes so that they only work on Java 8, or you can point the compiler at a copy of the Java 6 libraries so that the compiler won't see any of the new APIs (and then if the program compiles it should work on the older version).
That message is a warning and not an error. If you are absolutely sure you aren't using Java 7 or newer API methods then it can be ignored. On a larger project, it is much easier to let the compiler do the work to be absolutely sure, by using the javac command line option mentioned in the warning.