Hi,
if 10 dollars is desired from me, but I only have 9 dollars, then it is a dollar short for me. Is this understanding right? here, you are literally talking about money. Yes, you'd say 'I am a dollar short'.
However, you can also use the phrase figuratively. When you say 'Tom is a day late and a dollar short', you are not literally talking about money. Simply put, you are talking about his character and/or conduct.
Best wishes, Clive