Write an L2 rollup explainer (120-180 words)

An L2 rollup is a scaling solution built on top of Ethereum that makes transactions cheaper by moving most of the computation off the main chain. Instead of every transaction being processed directly on Ethereum, a rollup handles execution on a separate layer, bundles many transactions together, and posts a compressed summary back to Ethereum as a single record. This is why it makes Ethereum transactions cheaper. On the main chain, each transaction competes for limited block space and pays gas fees accordingly. With a rollup, that cost gets shared across hundreds or thousands of transactions that were batched together, so each individual user pays only a small fraction of what they would on Ethereum directly. The two main types are Optimistic Rollups, which assume transactions are valid unless challenged, and ZK Rollups, which use cryptographic proofs to verify batches. Both approaches reduce fees significantly while still settling on Ethereum and inheriting its security guarantees.

An L2 rollup is a scaling solution built on top of Ethereum that makes transactions cheaper by moving most of the computation off the main chain. Instead of every transaction being processed directly on Ethereum, a rollup handles execution on a separate layer, bundles many transactions together, and posts a compressed summary back to Ethereum as a single record. This is why it makes Ethereum transactions cheaper. On the main chain, each transaction competes for limited block space and pays gas fees accordingly. With a rollup, that cost gets shared across hundreds or thousands of transactions that were batched together, so each individual user pays only a small fraction of what they would on Ethereum directly. The two main types are Optimistic Rollups, which assume transactions are valid unless challenged, and ZK Rollups, which use cryptographic proofs to verify batches. Both approaches reduce fees significantly while still settling on Ethereum and inheriting its security guarantees.