expiration -> after this time the transaction can never be included in a block.
ref_block_num / ref_block_prefix -> this transaction can only in blockchains where the highest block_num%0xff has a
blockid that contains ref_block_prefix. Stated another way, this says the transaction can only be included in forks that build off of the reference block.
Taken together the transaction is only valid for the specified period of time and can be declared invalid if it isn't included in a block by the time the last irreversible block time is equal to the expiration.
EOSIO uses this information to prevent replay attacks on transactions.
delay_sec will cause the transaction to be executed a specified number of seconds after being included in a block. During this time the user can cancel it. Some permissions and contracts require a minimum delay period for security purposes.
netusage and cpu usage limit how much a block producer can bill the signers for the transaction, set to 0 to allow the producers to subjectively determine your resource usage.