Net debt is total debt minus the cash and short-term investments that could repay it immediately. A business with $20B of debt and $30B of cash has net debt of negative $10B, more firepower than obligations.
Negative net debt is common in tech and strong consumer brands, while heavily positive net debt is normal for utilities and industrials, whose infrastructure is financed with borrowing. Net debt is what enterprise value adds to market cap, since a buyer of the whole business inherits the debt and gets the cash.