Post-consumer waste is from products that have been used and then put back into the recycling stream. As far as paper products go, this could be newspapers, magazines, junk mail - any paper that has been used.
Pre-consumer waste is reintroduction of manufacturing scrap back into the manufacturing process. When paper gets trimmed during the manufacturing process, the ends that are cut off can be recycled and made into new paper products.
So my 100% recycled tissues came from 80% used recycled materials (but hopefully not used tissues) and most likely the other 20% was recycled manufacturing scraps. That makes sense.
The terms post-consumer and pre-consumer waste don't just refer to paper products. Any products that are made from recycled materials can contain both types of waste.