The short answer is that they're tokens, represented in a distributed ledger. They have value solely because of their scarcity.
The longer answer is that the mathematics behind this ledger (blockchain) is much more interesting than the ability to buy and sell tokens with real money. The oldest of the blockchains is Bitcoin, it's been running for around 15 years and has the highest scarcity value. It has zero intrinsic value, it has just risen in scarcity as more people have learned of its existence and joined in - as a sort of pyramid scheme..