What does ‘быть за’ and ‘быть против’ mean in Russian

To express your agreement and disagreement in Russian, it is enough to say ‘Я за’ and ‘Я против’. ‘Быть за‘ means to support an idea, an offer or even a person. ‘Быть против‘ means the opposite. 

— Давайте поужинаем сегодня в итальянском ресторане. = Let’s have dinner at an Italian restaurant tonight.

— Я за! = I’m in! / — Я против. = I mind.