Post-war cards refer to trading cards produced after World War II — a period that marks the birth of the modern hobby as we know it. This designation helps distinguish between the ultra-vintage pre-war era and the more accessible (but still highly collectible) post-war era.
Post-war cards were generally produced in much larger quantities than pre-war issues, making them more attainable for today's collectors. But don't mistake "more accessible" for "not valuable" — some of the most famous cards in hobby history are post-war: the 1952 Topps Mickey Mantle, the 1989 Upper Deck Ken Griffey Jr., and the 1948 Leaf Jackie Robinson, to name a few.
If you're collecting vintage but pre-war prices are out of reach, the post-war era is loaded with iconic cards at a wider range of price points.