The native currency on Optimism (used to pay gas, like ETH is used on Ethereum) is effectively ETH; but, as it isn't Ethereum, that ETH on Optimism has to actually live on Ethereum: it gets locked into a contract there which acts as a repository/reserve for all of the ETH being used on Optimism.
When you deposit ETH in this reserve on Ethereum you get credited the same amount on Optimism in the form of cryptocurrency IOUs (which we might call "OETH"), and you can later withdraw that money back to Ethereum, whereupon the OETH is destroyed and ETH is unlocked from the reserve contract.
The bug here (which I go into detail in in my post-mortem, along with another / different description of how these "bridges" work) was in the VM used for the smart contract behaviors on Optimism, which would mean you could arbitrarily replicate OETH (the IOUs for ETH).
For avoidance of any doubt: you couldn't use this bug to create an arbitrary amount of ETH/Ether, but the issue is that a lot of people call the money on Optimism--which is normally backed 1:1 with ETH--"ETH". (There is a discussion about what it should be called in the Ethereum chains database; I personally think what we need is a terminology for describing the full path whenever you have "ETH via an indirect path".)
