You can also calculated their book value, which is their assets minus their liabilities, but that is smaller than their market cap (as it should be).
Imagine if you bought a restaurant for $100 cash, and I bought an identical restaurant with $50 cash and $50 of debt against the business. Both restaurants are worth $100.
Another way to think about total enterprise value is that there are two groups of people who have a claim on the business: the equity holders and the creditors. In order to own the business free and clear, you need to buy both of their claims. The total equity claim is the market cap and the total creditor claim is the outstanding debt.