It depends on the context. Generally it is a preposition meaning 'on', but that is not to say that it is always best translated like that:

• Mae'r gath yn eistedd ar y bwrdd - The cat is sitting on the table.
• Un ar ddeg (= Un deg un) - Eleven ( the traditional un ar ddeg version is generally still used with time).

There are other uses of ar as well which you may come across from time to time.