If you select that option (and hit the "Save" button next to it), you will become the "thread
boost affiliate" for that thread. If someone
boosts that thread while you are the thread boost affiliate, you will earn an affiliate commission. Right now the thread boost affiliate commission is 50%.
Maybe you can create a thread about the awesome product X. Become the affiliate of that thread, and ask the people who sells X to boost your thread. If you manage to convince them to boost the thread, half of the coin is yours!