A percentage of a future transfer fee owed to a player's former club. If Club A sells a player to Club B with a 20% sell-on clause, and Club B later sells the player for 50 million, Club A gets 10 million. Smart selling clubs use these to profit from players they developed. Southampton's business model was essentially this for a decade.
"It is why a selling club celebrates a former player's 50 million move almost as loudly as the buying club: a fifth of the fee might be theirs."