Exactly what it means, DEBT!
Just like you and me, companies can accrue debt.
Companies may decide to take at loans for a number of reasons, typically debt will consist of bonds to investors, loans on property & equipment, etc.
The main thing to remember is that when a company has debt they also have an INTEREST BEARING EXPENSE until the debt is paid off. Just like a credit card.
Microsoft currently has approximately $1.2 billion in short term debt & $10.7 billion in long term debt.