SFN is a common line note below an item on a ticket. SFN is short for Something For Nothing, meaning that the waiter who rang in the order intends to give the diner something free of charge.
Servers can give out SFNs for various reasons - perhaps the guest had an unusually long wait for a table, maybe they are industry, a former employee, or some other form of VIP. They could also be regular visitors to the restaurant and the managers want to make sure they know they are appreciated.
Whatever the reason, it is important that SFNs are added to the ticket so that there is a record, but also so that the item doesn't get lost in the shuffle in an otherwise busy service.
SFN'd items tend to be the ones that don't involve large proteins or something where the food item itself is too expensive a cost for the restaurant to eat. A large format steak will probably not be SFN'd the way a carpaccio or tartare would. It's likely going to be an item that the kitcen is proud of and may have taken some serious time to prepare, but isn't a big hit in terms of food cost.