What do we mean by primitive material? How can we tell if a meteorite is primitive?

Primitive material was formed in the early stage of the solar system, before planets cooled off enough to differentiate elements of different density. It was not subject to great heat or pressure after it formed.

We can identify primitive meteorites by their composition, primitive meteorites are usually undifferentiated stones, with some metallic grains mixed in. Some primitive meteorites are darker, carbonaceous stones.