In blockchain, to burn a token is to make it unusable.  This is typically done by sending the token to an account with a nulled key (cryptographically inaccessible). However, since EOSIO allows 15 of 21 Block Producers to access tokens on an account that has nulled keys, it is best to burn tokens by using the retire function to remove the tokens from circulation completely. Further, since inflation is based upon the total supply, tokens burned by sending to a nulled account, but not those burned with the retire action, will still be included in the circulating supply.