To put it simply, a latch is the attachment of your baby's mouth onto your breast. Having a good, deep latch is one of the most important parts of breastfeeding, because it helps ensure that your baby can drink effectively from your breast.
Although breastfeeding is natural, it's also a learned experience, just like anything you're doing for the first time.
Although sucking is a part of breastfeeding, what you really want your baby to do is drink. And in order to drink effectively, there needs to be a good, deep latch.
Why? Because the nipple is just the delivery system for the milk. The area of the breast the baby needs to access or "milk" is just behind the nipple/areola, where milk gathers in the milk ducts. Once he's deeply latched onto your breast, he uses his tongue to milk the breast in a wave-like motion, which pulls the milk from this area into his mouth via the nipple. That's why you want to have more breast tissue in your baby's mouth near his tongue. To achieve that, your nipple shouldn't be centered in your baby's mouth but should be asymmetrical, with more areola showing near his top lip than the bottom. This is where newborn babies need our help.
Try to think of your baby latching onto your breast as taking in a large mouthful of breast tissue rather than just the nipple. We compared this to taking a bite from a sandwich, where you can help by compressing your breast in a U shape so it fits deeply into your baby's mouth.
Not only is a good latch important for your baby to access the most milk but it's also more comfortable for you. In fact, if you're having pain at the breast—not just discomfort, but real pain—it's often caused by a poor latch.
It's important to take the time to learn how to get a good latch, both what it looks like and how it feels. If you're experiencing painful nipples, this can often be solved by adjusting the latch.
Source: Heather Kelly is an International Board Certified Lactation Consultant (IBCLC)