What is a rollup?

Think of a store that ships its orders on a shared truck instead of one vehicle per parcel. That’s roughly how a rollup works: it executes many transactions on a second layer (Layer 2), bundles them into batches and posts to the main network (Layer 1), such as Ethereum, the data needed to check that they are correct. That way, the cost of posting is spread across more transactions.

The main blockchain doesn’t need to re-execute every transaction individually. It does, however, check the proofs or settle disputes, depending on the type of rollup. This link is the key difference from a sidechain, which relies on its own security mechanism.

How transactions are checked

In optimistic rollups, results are initially assumed to be valid, and there’s a window during which they can be challenged with a fraud proof. That’s why withdrawing to the main network through the native bridge may require waiting until that window closes.

In zk-rollups, each batch comes with a cryptographic validity proof that the main network checks. Despite the name, a zk-rollup doesn’t automatically hide your transactions. Proving that computations were done correctly and privacy are different things.

Why it matters

If you use decentralized apps or move crypto, a rollup can offer lower fees and faster initial confirmation. But an instant success message in your wallet isn’t always the same as finality on the main network. We consider it significant that capacity increases without handing all control to an independent chain.

What to watch out for

Security isn’t the same across all rollups. Bugs in smart contracts, upgrade permissions and reliance on a single central transaction sequencer can create risks or delays. If you move money through a bridge, check the destination network and how to get back. Having the same wallet address on two networks doesn’t mean balances move between them automatically.