Just tell the person that bitcoin is a digital currency that is not being regulated by any Central Bank and it can be transfer through internet to any body in any part of the world just within some seconds
But I think it is not so easy to convince a person and specially a newbie. If you are going to explain bitcoin to a newbie, it is too much important that you must also have good knowledge about bitcoin. you can also give him a practical example of yourself, through which we can more easily convince a person about bitcoin.