This is the 963rd most frequent Japanese word.
反対
Opposition; objection; the opposite.
In this sentence, '反対' is used as a noun to express disagreement or opposition to an idea or proposal.
私はその提案に反対です。
I oppose that proposal.
Here, '反対' is part of a verb form '反対して', meaning to oppose or disagree with something actively.
彼の意見に反対して、別の意見を述べました。
I opposed his opinion and expressed a different one.
In this example, '反対' means 'opposite' as in 'the lane on the opposite side' or 'the opposing lane' contextually.