Unlevered Free Cash Flow is a company's free cash flow before interest payments.
Unlevered FCF = EBITDA - Capital Expenditures - Changes in Net Working Capital.
Unlevered free cash flow is often useful when an investor is interested in acquiring a company. The investor would aim to change the company's capital structure by paying down existing debt, thus "levering up". The company's unlevered free cash flow is important as it would indicate to this investor the company's free cash flow if their existing debt was paid down.