For many sport backpacks, waterproof property is necessary, especially for hiking backpack. Some backpacks are made from waterproof fabric, but zippers are the common woven type. Actually, this kind of backpack is not really waterproof because water can get into backpack easily through the woven zippers.
At this occasion, you may need a waterproof zipper. In fact, there are several kinds of waterproof zippers. One kind is the zipper covered with hot melt adhesive on one side. Another kind of zippers is made from TPU coated webbing.
The outer coated TPU protect inner woven webbing from water, air and dirt, which makes TPU coated webbing a waterproof and cleanable material. When TPU coated webbing combines with zipper teeth, a waterproof zipper comes out.
Ps. Some pictures of this article is from internet.