Overly simplistic and assuming you are talking about the US: So you spent 80M making a movie but for whatever reason it never makes it out of production, you never see any return on this investment. But overall this year you made 100M from something else. When you do your taxes, you can “write off” this 80M spent as a loss, so you only have to pay taxes on the remaining 20 million. It doesn’t deduct 80M from taxes owed.