Enterprise value is

  market cap + debt - cash

It reflects how much it would cost to buy a company out.

It's the valuation of the company based on number of shares and share price less actual assets (cash in this case). What is left is the value investors see in the company simply by executing its business.

It's basically market cap plus the debt, minus the cash. So $57 billion minus $14 billion in cash. Try to think of it as the actual amount somebody would loose from pocket while buying Facebook. While they will pay $57 billion, they will get $14 billion cash from Facebook, effectively spending $43 billion.

Take a look at the numbers he mentions:

    valuation $57.5bn
    cash on the books $14bn
    enterprise value $43bn

If you look real closely, you will see that 57-14=43.

Enterprise value "is a sum of claims of all the security-holders: debtholders, preferred shareholders, minority shareholders, common equity holders, and others." (source: http://en.wikipedia.org/wiki/Enterprise_value)

He's looking at market price (number of shares times share price) and subtracting cash on hand. That would be the implied value of the operating business.