A colloquial expression meaning "that's true" or "of course" used to agree or state that something is obvious.
A: 彼は一晩中勉強していたよ。B: そりゃそうだ、試験前だからね。
A: He studied all night. B: Well, of course — it's before the exam.
給料が低いなら辞めるのも当然だよ。そりゃそうだ。
If the pay is low, quitting is natural. That's obvious.
彼女が怒るのは無理もない。そりゃそうだよ。
It's no wonder she's angry. That's understandable.