Ethereum tokens are simply digital assets that are being built on top of the Ethereum blockchain. They benefit from Ethereum's existing infrastructure instead of developers having to build an entirely new blockchain.
Ethereum is a platform that can be used to create any arbitrary smart contract including smart contracts that represent digital assets called Ethereum tokens. This is similar to the App Store providing a platform for iOS apps with some apps issuing their own digital currencies that are used within the game or system. However, with Ethereum there is no centralized entity like Apple that controls what gets added to the App Store. Anyone can create a token on top of Ethereum.