It's a price that "does not reflect legitimate forces of supply and demand". One of the canonical examples of an artificial price is spoofing, where you set up fake sell orders to trick people into thinking a stock is crashing so you can buy it at a discount.

The "legitimate forces of supply and demand" seems to be the critical part there. GME stock didn't go up because /u/DeepFuckingValue bought so much of it, it went up because everyone else did. In August 2019 GME had a market cap of $300 million so there was no way /u/DeepFuckingValue buying some hundreds of thousands worth had any real effect on its own.