Mostly digital assets that can transact on a particular Blockchain network.
Think of it as petrol for your car.
For eg, to use the Stellar blockchain network, you need Lumens (XLM).
Just like petrol, these currencies are designed to have a limited supply to ensure that when the demand increases, the value of the currency increases as well.
What’s interesting though, is that for every different brand of car, you need to use that particular brand of petrol.
Those Lumens cannot directly be used in the Ripple Blockchain network.