Burn the SPT means to send the SPT to Hive account@null. Null is an account no-one has the keys for so anything sent to it is "burned". Doing this action lowers the SPT in circulation. The above screenshot shows that I used 3436 SPT to promote the burn post. The action of using the promote feature on the Splintertalk frontend sends the tokens used to promote to Hive account@null.