An expression used to agree with someone's remark — meaning something like "that's true" or "you have a point." Common in conversation.
A: 今日は人が多すぎるね。 B: それもそうだ、もっと早く来ればよかった。
A: There are too many people today. B: That's true — we should have come earlier.
彼が忙しいのを知っているなら、頼まない方がいいよ。— それもそうだね。
If you know he's busy, you shouldn't ask him. — That's true.
予算が足りないなら計画を縮小するしかない。— それもそうだ。
If the budget isn't enough, we have no choice but to scale back the plan. — That's true.
君の言うことには一理ある。— それもそうだ。
There's some truth to what you say. — That's true.