BIAB stands for 'builder in a bottle', which is also known as builder gel. This range of nail products are specifically designed to add strength and structure to your natural nails. If you're familiar with the brand The Gel Bottle, you'll know that they're credited with developing BIAB.
It's a diverse tool, too. It can be used to add length to your natural nails, creating longer tips. You can also layer BIAB on top of your natural nails to give them strength and prevent breakage.