A casual expression meaning "don't worry about it" or "never mind"; a blunt imperative form of reassuring someone.
ミスしても気にするな、次があるよ。
Don't worry about the mistake — there will be another chance.
遅刻のことで気にするな。問題ないから。
Don't worry about being late. It's not a problem.
気にするなと言っただろう？もう話を変えよう。
Didn't I tell you not to worry? Let's change the subject.
友達が気にするなと励ましてくれたが、彼はまだ不安そうだった。
A friend told him not to worry, but he still looked uneasy.