Interest expense deduction is defined as a borrowing expense that a taxpayer can claim to reduce their taxable income. There are many types of interest that can be tax-deductible such as mortgage interest, student loan interest, investment property loan, interest on some business loans.
For example, if an investor has a 30% marginal tax rate and has $10,000 in tax deductible income, they would save $3,000 in taxes. Effectively that $10,000 loan only cost $7,000.