Tokens are used very differently in different cryptocurrencies.
Here is a real-world example: if I give you a pound coin, it is a token. We just call it a pound because that is the currency used in the UK. Some currency authors, to avoid confusion with currencies, will use the word “token” instead of “coin”. It’s just a medium of exchange.
What tokens mean for the blockchain depends on the system you are using them in. If it’s a currency, then it’s a coin that is traded. If it’s something like an energy trading scheme, then it could be something like a kilowatt hour of energy. So you need tokens to exchange information between parties.