A factory set is a complete set of cards packaged and sold in a single box — no randomness, no pack-ripping, just the whole checklist ready to go. Instead of buying random packs stuffed into retail or hobby boxes, you get every card from the set in one purchase.
Factory sets first showed up in the 1980s, back when hand-collated sets from mail catalogs were hugely popular. They're a set builder's dream — skip the hunting and get straight to completion. They're also a great way for new collectors to jump into the hobby with a full set from day one.