A phrase telling someone not to overexert themselves; 'don't overdo it' or 'take it easy'.
忙しそうだけど、無理しないでくださいね。
You seem busy, but please don't overdo it.
怪我をしているなら、無理しないで安静にしていてください。
If you're injured, don't push yourself and rest.
テスト前だからといって無理しないで、計画的に勉強しよう。
Don't overdo it just because the test is coming; study in a planned way.