To my understanding, this means that the resin is pushed away, by the oxygen, from the areas that's being printed, so that the UV light only hits that part.
Unlike SLA, where beams of UV light is projected to different areas to cure the resin, this CLIP does it so that the UV light is continuously projected on the whole area, and the oxygen determines where the resin is cured.
When this is done in a continuous motion there won't be any layers although, in a way, the parts are still being built layer per layer.