The Briar is a flower added by Thaumic Bases. It can be harvested when fully mature for 1-4 Briar Seed Bags, which can be crafted into Rosehip Syrup.
Plants are known for their medicinal uses. However, thaumaturges often do not care about this side of nature. You do care. You have managed to bring the Rose bush back to it's wild state, thus creating a Briar plant. Briar is a 2 block tall plant that will grow pretty fast. Once grown it's seed bags can be collected for a later usage. They can be combined with sugar in your Arcane Workbench to create a Rosehip Syrup. This item acts just like a better milk - it stacks up to 16, and it is known for helping thaumaturges with some warp effects. It will not work with some purely mental effects though...