What is money really?
Money is, in essence, debt. More broadly, it's a system of clearing and credit. That entry in the computer means that the bank owes you \$1000, which is worth whatever others are willing to exchange for that sum— one way of thinking about it is that if you have \$1000, you have a general claim on the rest of society for \$1000 worth of whatever society produces. 

The fact is that most money is in the form of loans (debt), not currency. This is well-explained in a recent Bank of England paper.