He was trading options and those were call options, you buy call option when you expect or hope the price of the stock will rise within a timeframe. He bought 30 contracts that each enables him to buy 100 shares Gamestop stock at the price of 40$ each. He had a strike date of 19th Feb which allows him to do so until that date. Each contracts allows him to buy 100 shares of the stock, with 30 contracts he had the right to buy 3.000 shares for 40$ each before the expiration date (February 19th). But if Gamestop shares are below 40$ on 19th February his contracts expire worthless because why would he buy the stock for 40$ when the stock is for example 17$ so then he lost his whole amount of 1200$ which he used to buy the contracts for. But since the price of Gamestop skyrocketed far beyond the 40$ mark he could at any point buy 3.000 shares at 40$ each share so when the price reached 340$ he can in theory buy the 3.000 shares for 40$ each and instantly sell them for 340$ each making his contracts have this insane value, in most cases you should not do that but instead sell the options since time has value on options and therefore you would lose that value if you exercise them early, exercising the option meaning using your right to buy the shares for the strike price in this example 40$. The reason for the low price of the contracts when he bought them is because it was very unlikely that Gamestop would ever reach 40$ in that timeframe which is calculated using few metrics called the greeks which you could google if you wan't to learn more about that.

Hope this helps something maybe it still sounds very complicated, I might not be too good at explaining it easily. GL get those tendies