Note: this is a feature in the release process. Check to see current production network status.
The Internet Computer blockchain can stretch the execution of a single transaction across multiple blocks. This allows very-long running smart contract computations to be initiated.
By contrast, traditional blockchains that host smart contracts, always need to finish processing a transaction within a single block. In practice this means that when a smart contract function is invoked by a transaction, it must complete execution within the per-block "gas limit" (gas is the equivalent of cycles on the Internet Computer).
The Internet Computer is designed to allow any online system or service to be built using smart contracts called canisters, to support a "blockchain singularity." This means that when a transaction invokes a smart contract, the computation involved must be able to run for a very long time if required, and consume as many computational cycles as needed.
This means that on the Internet Computer, a smart contract is capable of performing complex AI calculations, or applying a graphical filter to an image, say — although there is still a "max cycles limit," it's just very high.
Multi-block TXs are made possible by a broader aspect of the Internet Computer's design, called Deterministic Time Slicing (DTS).