An expression meaning 'it doesn't matter' or 'no problem'; widely used in everyday speech (often casual: 'I don't mind' / 'it's okay').
そんなに遅れても構わないよ。
It's fine even if you're that late.
「窓を開けてもいいですか」「はい、構いません。」
"May I open the window?" "Yes, that's fine."
彼の服装がどうでも私は構わない。
I don't care what he wears.
怪我がないなら、それで構わない。
If there's no injury, that's fine.