system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly were they doing that was illegal in The Wolf of Wall Street?
gpt
The illegal thing they were doing is referred to as a "pump and dump" scheme. 

There were these things called penny stocks. When the average person could not afford a stock selling at $30-$1,000 a share, there were these stocks which sold from $.01- $15 a share. 

What they were doing was buying up all or most of the shares of the penny stocks, thus increasing demand and price. Jordan was selling his clients these penny stock by telling them the price was going up, but failing to tell them it was going up because he was buying the shares. To get Jordan Belfort to buy their shares for them, as their trusted broker, Jordan would charge them a fee. When the client wanted to cash out, they could'nt because there was no one to buy the shares of penny stocks from them. 

Whats illegal about this is that Jordan Belfort committed fraud by charging a fee to trade shares which he was inflating and getting money by being the one who is selling the shares.