whatever rewards @burnpost gets will get sent to @null.
@null is an account controlled by no one, so once the coins are sent there, they can't be sold anymore. Therefore they are burned. This means less steem will be sold on the market, it reduces the supply of steem forever.
Correct, although not quite complete. The reason the burnpost rules are a little more complicated is the procedure is different when SBD is worth more than $1 (USD). In order to help the peg, the SBD rewards are sold instead of burned, and the burning is delayed, but all rewards (and proceeds from selling the SBD rewards) are still eventually burned.
At present SBD is not worth more than $1 so this part doesn't currently apply.