None of these explanations have ever covered what the hell WETH is, or the reason the person was trying to exchange it. Seems like there's still a lot of implicit assumptions in these explanations.
WETH is short for “wrapped Ether”. It’s a token issued on the Ethereum blockchain. This token conforms to the so-called ERC-20 standard, which is — essentially — the API of the token. Ether itself does not conform to the ERC-20 standard, which is why wrapping it is necessary to e.g. exchange it for other ERC-20 tokens via a smart contract designed for this purpose (confusingly called a “swap”).
