but come across one thing which I do not really understand. What does [Int * (1 - t)] mean in this context? I understand that it’s interest expense but why multiply it with the non-tax rates?
The easiest way to see this, in my opinion, is to compare two companies that are essentially identical except for capital structure, so they will have the same FCFF:
Company 1:
Revenue = 1,000, all cash
Operating expenses = 700, all cash
Depreciation = 100
Interest expense = 0
Tax rate = 30%
FCInv = 50
WCInv = 0
Company 2:
Revenue = 1,000, all cash
Operating expenses = 700, all cash
Depreciation = 100
Interest expense = 20
Tax rate = 30%
FCInv = 50
WCInv = 0
Calculate FCFF for Company 1. Then calculate FCFF for Company 2, knowing that it has to be the same value as for Company 1. You should see pretty quickly why you add interest net of taxes.