Hello. tottallyoff is right. It simply means that someone has not got much money, temporarily. Sometimes we say (in UK at least) "I'm a bit short at the moment", and leave out the "of money" altogether, because it's understood.
Would you mind paying for the coffees? I'm a bit short of money - I haven't been to the bank yet.
He couldn't afford to buy much as he was a little short of money.