Resin 3D printers use resin-based photopolymers. These are liquid materials that react by solidifying when they are hit by a powerful laser or light source.
There are two ways to "activate" the photopolymerization reaction; one is to use a laser to "draw" the layers of the object. This is called pure stereolithography (SLA). Another is to project a whole layer of the object using a digital light projector. This process is called digital light processing (DLP).