Resin is the binding ingredient in paint, the component that forms the film after the paint dries. It determines how well the coating resists fading, cracking, and peeling, which is why resin quality varies noticeably between paint grades.
This is where premium paints earn their price. Cheaper paints cut resin content and substitute fillers, which shows up later as faster fading and shorter life. On exteriors especially, the binder is doing the heavy lifting all year, so choosing a quality paint with higher resin percentage makes the paint job last longer.