It's only when we truly know and understand that we have a limited time on earth - and that we have no way of knowing when our time is up, we will then begin to live each day to the fullest, as if it was the only one we had.

This quote is saying that when we realise how short our lives can be, and how unpredictable life is, we will use our time more wisely and intentionally. We will make sure we make the most of each day, as if it was the only day we had left. This means living fully and appreciating everything we do, as if it were our last day on earth.