A token is a digital asset created using the rules of another blockchain's programming environment — most commonly Ethereum's ERC-20 standard — rather than running on its own dedicated network the way Bitcoin or Ethereum's native coin does. This makes tokens relatively cheap and standardised to create.
Tokens can represent all sorts of things: a currency-like asset, a share of voting rights in a project (governance), access to a service, or a claim on a real-world or digital asset. The word "token" describes how the asset was built, not what it's meant to represent.