It means that your agency agrees with your client that he can use and publish your pictures for a certain amount of time, a certain area and a certain platform. If the client wants to extend the amount of time, extra areas and wants to use other platforms, he has to buy new rights. So, there is a huge possibility that you did a shoot once but get payed again and again for the rights of your pictures and videos. It is the agency’s obligation to take care of the buy-outs. The buy-out is usually put on top of the model’s salary.