I think what it means is that if I break your car and you sue me to get the money back from me, I can pay you in bitcoin and you'll have to accept it, i.e. cannot claim that I haven't repayed you for the damage. In all countries were Bitcoin is not an official currency (so pretty much everywhere) you could claim that even though I payed you in Bitcoin, it doesn't settle the debt and you reject the payment and ask me to pay you in an official currency.