noun
complete agreement; strong approval
Used to express wholehearted agreement with a proposal or opinion. Often used in the phrase 大賛成です.
その計画には大賛成です。
I'm all for that plan.
彼の意見に大賛成だ。
I completely agree with his opinion.
賛成 means 'agreement' or 'approval', while 大賛成 is a stronger, more emphatic version meaning 'complete agreement'.
Compound of 大 (だい, 'big, great') + 賛成 (さんせい, 'agreement').