It's talking about how the invention of the printing press allowed the distribution of written works (ideas) to a much wider audience. It's considered one of the greatest inventions of all time.
Previously the majority of people would have exchanged ideas by talking to each other. This can lead to subtle - or more significant - changes over time. One person can also only remember so much.
The printing press allowed ideas, concepts, philosophies, literature, diagrams, etc to be distributed in the exact way that they were originally created/conceived. Sort of like the impact that the internet has had these days.
The ideas aren't actually copying themselves; it's figurative. But, they are being copied exactly and distributed with the exact intent of the original author, rather than by word of mouth - or limited handwritten versions.