Getting back to this, if you're interested in exactly what happened here.
Axie Infinity is a cryptocurrency-based "pay to earn" game. The "earning" in this case means, hypothetically at least, that you can trade assets earned in-game for some amount of the cryptocurrency Etherium.
Etherium is too expensive to use directly for continuous small transactions like these - every single transfer of Etherium has a fee attached, called "GAS", whether you're moving it between your own wallets, actually "buying" something with it, transferring it to an exchange, or whatever; and these GAS fees are so high that spending Etherium directly on anything worth less than several hundred dollars is usually impractical. And as if that weren't enough, Etherium, like all proof-of-work blockchain cryptocurrencies, is so ponderously slow that because of its volatility, the actual value of this or that amount of Ether can change - like, substantially - between when a transaction starts and when it's completed, meaning that a buyer or seller could end up getting shafted mid-transaction and there's not really anything they can do about that.
One solution to this problem is "side-chains", which are like little pocket-crypto-universes. The side-chain, like a bank, possesses a certain amount of the base cryptocurrency - Etherium in this case. People can trade and use the currency a lot more quickly and with low or no transaction fees on the side-chain, because these transactions aren't "official" as far as Etherium's actual blockchain is concerned, and they don't use Etherium's validation process. So users on the side-chain aren't really trading Etherium, they're trading "Etherium" - or "wrapped Etherium". When a user on the side-chain decides to "cash out", the side-chain's "official" Etherium wallet pays the user 's official Etherium wallet however much the user is wanting to "withdraw". And this being an official transaction comes with the normal Etherium fees (and slowness, etc). So what you want to do is handle all the small-time transactions on the side-chain and only "cash out" when you have a large enough amount to make it worth the cost of doing so.
Axie Infinity's developers created a side-chain just for their game, called Ronin. The game has a few in-game currencies and NFTs which can be sold for "Etherium" in the Ronin bridge which can in turn be traded into real Etherium.
What happened in this "hack" is, the thieves gained control of the small number of accounts that the Ronin side-chain used to validate transactions, and used them to "validate" the transfer of over $600m worth of Etherium out of Ronin's official Ether wallet to the thieves' official Ether wallet.
So Ronin has no Etherium left in its possession. That means that you, a hypothetical Axie player, may have say 20 "Etherium" saved up in the Ronin side-chain, but you actually have zero Etherium. You cannot cash out of the side-chain because Ronin has no Etherium to "officially" transfer to you. If you hadn't cashed out before the heist, all the time you spent grinding in this game to "earn" money has been wasted.
