Tax rules often allow faster write-offs of equipment than accounting rules do. The company pays less tax now and more later, and the gap sits as a liability. For a business that keeps investing, that liability rolls forward indefinitely and behaves like interest-free funding.
Analysts sometimes exclude it from total-debt for exactly that reason, while noting that a company that stops investing will eventually have to pay.
Example: Northwind Tools carries $40M of deferred tax liability arising from claiming accelerated depreciation on its $520M of property-plant-and-equipment. It has grown every year for a decade.
Related: deferred-tax-asset, income-tax-expense, total-debt, effective-tax-rate