The third aliyah continues after Aharon prepared offerings on the altar in the previous aliyah. At this point a fire comes down from G-d and miraculously consumes the offerings. When the people witness this miracle, they praise G-d and prostrate themselves down on their faces.
After that, two of Aharon's sons, Nadav and Avihu, make unauthorized offerings, and this time a fire came down from G-d and consumes them, and they pass away. Moshe orders their burial.