It is a statement built on incorrect logic or reasoning that is false.
e.g. I start with some amount of money I make a 10% loss and then a 10% gain, I now have the same amount of money I started with. This is obviously not true: if I have $10 and lose $1 (so I have $9) I have lost 10% of my money, later on I make $0.90 so I have made a 10% gain (so now I have $9.90, $0.10 less than I started with)