What does ]] means in syntex.

Hi,

below is syntex of commit from http://technet.microsoft.com/en-us/library/ms190295(v=sql.90).aspx

COMMIT { TRAN | TRANSACTION } [ transaction_name | @tran_name_variable ] ]
[ ; ]

what does ]] means, please provide link and line no where it is decribed.

yours sincely