Waterproof/ Water-resistant- Unlike fragrance-free or alcohol free which are pretty self explanatory, waterproof/water resistant is a bit more complex. Even an industrial strength sunscreen will not be able to deliver on its promise if its washed off by perspiration or a dip in a pool. So waterproof/water resistant ( and its cousin sweatproof) means that this sunscreen will have some staying power on the skin. However its ability to withstand moisture is not permanent- which is why the usual recommendation is to reapply sunscreen every few hours.
Bottom Line: waterproof/water resistant is an important feature if you’re using sunscreen around a pool or beach or in hot weather. When its cool and dry, water resistance is not a key factor– but remember as long as there is sunlight, even in winter, you need a daily sunscreen.