It literally translates to "big/much good fortune", conveying a slightly more formal and well-wishing tone than just "bye".
Here are some other informal ways to say goodbye in Welsh:
* Cymhwys - This is a more casual goodbye, similar to "later" or "see you later".
* Hwyl fawr i ti - This is a more personal goodbye, meaning "much good fortune to you".
Remember that the best way to say goodbye in Welsh depends on the context and the person you are speaking to.