For SPT and other HIVE related cryptocurrency burning it would mean sending it to the HIVE account @null. No-one has the keys/passwords for the @null account so anything sent to @null can't be recovered.

Burning reduces the amount of the token in circulation. A general rule is the less of something there is, the more valuable it can become. Burning alone won't give a token value as tokens/coins usually need a use case if they are to sustain any USD value.