At first I thought it meant every time you hit one of Moon Lord's targets it cools down before you can hit it again. But what it really means is you can't stand on spikes or in lava anymore and be immune to his attacks. If you are blinking because you got hit from something else, you can still get hit by Moon Lord. But if you are blinking because you got hit by Moon Lord, you will still be immune to his damage for some time.