From my understanding, what that means is for some cases, if some other clubs decide to pay huge fee to buy the player, the club that has the buy-back right can block the sale, and the other club can't do nothing. For the sale to go through you need 3 clubs, and the player to agree.
Juventus striker Alvaro Morata has dropped his biggest hint yet that he could turn down former club Real Madrid when they activate the buyout clause that was included in the deal that took him to Turin...
bleacherreport.com
"Buyback options aren't mandatory. Every player has the right to turn down the proposed move back."